Integrated internal combustion engine control system with high-precision emission controls

ABSTRACT

An integrated internal combustion engine control system in combination with an automotive emission control system, comprises an exhaust-gas recirculation valve (EGR valve) employed in an EGR system. A target EGR amount is calculated as a function of an intake-air flow rate measured by an air-flow meter and a target EGR rate based on engine operating conditions such as engine speed and engine load. The opening of the EGR valve is adjusted by a command from a control unit so that the target EGR amount is attained. The target EGR rate is preferably calculated depending on an intake pressure as well as the engine speed and the engine load. In combination with the above-mentioned EGR control, a throttle valve disposed in the induction system is effectively controlled in response to an actual valve lift of the EGR valve and a differential pressure between an exhaust pressure and the intake pressure. A fuel-injection amount is accurately controlled in consideration of a variation of an excess-air factor. To compensate a phase lag a predetermined advance processing is made to a signal from a typical air-flow meter, and then the advance-processed signal is effectively signal-processed to invert a reverse-flow component in the output signal.

This application is a division of application Ser. No. 08/678,590, filedJul. 12. 1996, now U.S. Pat. No. 5,918,582.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automotive emission control systemsuitable for use in internal combustion engines, and specifically to anelectronic centralized engine control system which performs variousengine controls, such as an exhaust-gas-recirculation control, a fuelinjection control, a precise detection of flow rate of intake air drawninto an intake manifold or the like.

2. Description of the Prior Art

As is generally known, in order to minimize or eliminate atmosphericpollution from automotive vehicles, there have been proposed anddeveloped various automotive tune-up and exhaust-emission controltechniques. For example, an exhaust-gas-recirculation control system,often abbreviated simply as an "EGR system", is used to reduce nitrogenoxide (NOx) emissions from exhaust gases of the internal combustionengine. On late-model diesel-engine cars, the EGR system is almost allemployed to decrease formation of NOx. In typical EGR systems, a targetEGR rate or a target EGR amount is determined depending on engineoperating conditions, namely engine speed and load on the engine. Theengine load can be generally estimated by a fuel-injection amount, anopening of an accelerator (an accelerator pedal position) or the like.One such prior art EGR system has been disclosed in Japanese PatentProvisional Publication No. 58-35255. In an EGR system incorporated indiesel engines, it is desired to properly and precisely control theamount of exhaust-gas recirculation (EGR) or the rate of EGR, in atransition state for example a transition from normal straight-aheaddriving of the vehicle to heavy acceleration, or in case of changes inair density resulting from changes in environment from low-land drivingto high-land driving even under constant engine speed and engine load.Also, in a diesel engine with a turbocharger, there is the additionalproblem that the acceleration versus super-charged induction pressure(often called boost pressure) characteristics is affected by degradationof lubricating oil for lubrication of the same turbine-wheel shaft asthe compressor-pump rotor. As is well known, in the event that theamount of EGR becomes excessively great, black smoke and particulatestend to increase. On the other hand, in the event that the amount of EGRis excessively less, the combustion temperature cannot be loweredsatisfactorily owing to less inert exhaust gases recirculated, and thusthe amount of NOx emissions cannot be sufficiently reduced. Particularlywhen quickly accelerating, the fuel-injection amount tends to be rapidlyincreased, and thus an excess-air factor tends to be greatly lowered,thereby resulting in increase in emissions of smoke and particulates Inorder to avoid the undesired lowering of excess-air factor, resultingfrom the rapid increase in fuel injection amount when quicklyaccelerating, the exhaust-gas recirculation would be intendedly cut. Theconventional EGR system could not timely perform the EGR-cut operationin the previously-noted transition state. The improper EGR-cut timingsproduce the increased amount of nitrogen oxide emissions (in case oflack of the EGR) or the increased amount of smoke and particulates (incase of excessively increased amount of the EGR). Specifically, in thecase of an engine with a turbocharger, there is a greatly increasedtendency for the previously-described improper EGR-cut timings to occurdue to fluctuations in the acceleration versus super-charged inductionpressure characteristics resulting from degraded lubricating oil. Toensure the EGR control or to reduce NOx emissions in the transitionstate such as during acceleration, the prior art EGR control system isequipped with an induction-air throttle valve and/or an exhaust throttlevalve to properly adjust the differential pressure between the intakepressure and the exhaust pressure and consequently to adjust the EGRrate toward a target EGR rate. For example, Japanese Patent ProvisionalPublication No. 60-219444 has taught the provision of an EGR controlwhich is in dependent on acceleration (or a rate of change in engineload). According to the EGR control disclosed in the Japanese PatentProvisional Publication No. 60-219444, an exhaust throttle valve isshifted to its fully-open position when the rate of change in engineload is held greater than a predetermined threshold for a preset periodof time. However, in case of an engine with a turbocharger, the optimalEGR rate would vary depending on degradation of lubricating oil as wellas the engine load. Japanese Patent Provisional Publication No.60-222551 has taught the provision of an exhaust throttle valve controlbased on a back pressure measured upstream of the exhaust throttlevalve. According to the Japanese Patent Provisional Publication No.60-222551, the opening of the exhaust throttle valve is adjusteddepending on the deviation between the back pressure measured and atarget back pressure which is predetermined by both engine load andengine speed, such that the measured back pressure is adjusted towardsthe target back pressure. As may be appreciated, it is troublesome toprecisely preset control characteristics of openings of the intakethrottle and/or the exhaust throttle, because the controlcharacteristics are affected by characteristics of an EGR control valve,different operating requirements of the engine and the like. In order toavoid excessive lowering of excess-air factor during acceleration of thevehicle, Japanese Patent Provisional Publication No. 58-138236 hastaught the stepwise increasing adjustment of a fuel injection amountfrom the time when the vehicle begins to accelerate. Actually the fuelinjection amount and/or the fuel-injection timings must be varieddepending on the presence or absence of exhaust-gas recirculation (EGR)or on the EGR rate. In the system disclosed in the Japanese PatentProvisional Publication No. 58-138236, assuming that the fuel-injectionamount and timings are adjusted to meet in the presence of the EGRduring acceleration, the fuel-injection timing tends to delay in absenceof the EGR, thus making a sacrifice of an acceleration performance. Incontrast to the above, assuming that the fuel-injection amount andtimings are adjusted to meet in the absence of the EGR duringacceleration, the excess-air factor tends to be excessively lowered inpresence of the EGR, thus increasing exhaust emissions such as blacksmoke and particulates. As appreciated, it is important to preciselydetect or measure a flow rate of intake air or induction air which isdrawn into an intake manifold. As is generally known, on late-modelcars, a precise measurement of intake air is required to determine afuel-injection amount in case of an electronically-controlledfuel-injection system for gasoline engines, and to determine a maximumfuel-injection amount in case of an electronically-controlledfuel-injection system for diesel engines. In recent years, a hot-wiretype air-flow meter is widely used to detect the flow rate of intake airflowing through the air-intake pipe disposed just downstream of an aircleaner. The hot-wire type air-flow meter is inexpensive and has arelatively wider dynamic range for flow-rate measurement. Owing to aso-called valve overlap during which the open periods of both intake andexhaust valves are overlapped, the intake valve opens from beforecompletion of the exhaust stroke, that is, prior to the top dead center(T. D. C.) position and the exhaust valve remains open after the T. D.C. position. During the valve overlap, there is a possibility ofback-flow or reverse-flow of some of intake air drawn into theintake-valve port. Particularly in case of a low flow rate of intake airor a high engine load, there is a tendency for pulsation flow of intakeair or pulsation of the manifold pressure to occur. The previously-notedconventional hot-wire type air-flow meter can measure the flow rate ofintake air but not detect directions of the intake-air flow. The flowrate of air flowing from the intake-valve port back to the intakemanifold would be measured erroneously as a positive flow rate. Thus, incase of occurrence of pulsation flow resulting from a low flow rate ofintake air, there is a tendency that the measured value of intake airmay be increased as compared with the actual flow rate. Theerroneously-measured flow rate of intake air may exert a bad influenceon the fuel-injection control, and whereby the engine performance or thedriveability of the vehicle may be degraded. In diesel engines, such ameasurement error results in deterioration in an exhaust-emissioncontrol performance, because the target EGR rate is usually determineddepending on the differential pressure between the exhaust pressure andthe intake pressure (or the manifold pressure). For example, in the casethat the measured value of the air-flow meter exceeds an actualintake-air flow rate, the EGR rate is set at a greater value than anoptimal EGR rate, thus increasing exhaust emissions for exampleparticulates. Conversely, in the case that the measured value is lessthan the actual intake-air flow rate, the EGR rate is set at a lessvalue than the optimal EGR rate, thus increasing the amount of NOxemissions. In these cases, the emission control performance tends to belowered. Also to avoid an erroneous measurement of the intake-air flowrate, occurring due to pulsation of the manifold pressure during highengine load, Japanese Patent Provisional Publication No. 57-56632 hastaught the use of an estimate of intake-air flow in place of a measuredvalue of the hot-wire type air-flow meter during the high engine load,i.e., when the throttle opening exceeds a predetermined threshold value.The estimate of intake-air flow is preset on the basis of both athrottle opening and an engine revolution speed. It is desirable thatthe deviation (the error) between the actual flow rate of intake air andthe estimate of intake-air flow is less as much as possible. However,when there are changes in air density due to a change in drivingcondition from low-land driving to high-land driving, thepreviously-noted deviation tends to increase. The increased deviationmay exert a bad influence on an accuracy of the fuel-injection controlor the EGR control.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an improvedautomotive emission control system for internal combustion engines whichavoids the foregoing disadvantages of the prior art.

It is another object of the invention to optimally tune up an internalcombustion engine by enhancing an exhaust-emission control performancewithout sacrificing an acceleration performance.

It is a further object of the invention to provide an automotiveemission control system suitable for internal combustion engines,particularly suitable for diesel engines with a turbocharger, which iscapable of ensuring an optimal exhaust-gas recirculation control, anoptimal fuel injection control and a precise detection of flow rate ofintake air.

In order to accomplish the aforementioned and other objects of theinvention, an integrated internal combustion engine control system incombination with an automotive emission control system, the enginecontrol system comprises an exhaust-gas recirculation valve employed inan exhaust-gas recirculation system for sending some of exhaust gas backthrough an internal combustion engine, air flow rate detection means fordetecting a flow rate of intake air drawn into the engine,engine-operating-condition detection means for detecting an operatingcondition of the engine, target exhaust-gas recirculation rate settingmeans for setting a target exhaust-gas recirculation rate depending onthe operating condition of the engine, target exhaust-gas recirculationamount setting means for setting a target exhaust-gas recirculationamount as a function of the flow rate of intake air and the targetexhaust-gas recirculation rate, and valve-opening control meansresponsive to the target exhaust-gas recirculation amount forcontrolling an opening of the exhaust-gas recirculation valve. Thevalve-opening control means may comprise a command exhaust-gasrecirculation amount setting means for setting a command exhaust-gasrecirculation amount (Tqec) by performing a predetermined advanceprocessing with respect to the target exhaust-gas recirculation amount(Tqec0), and a controlled variable setting means for setting acontrolled variable (Liftt) of the opening of the exhaust-gasrecirculation valve depending on the command exhaust-gas recirculationamount. A time constant of the predetermined advance processing is setdepending on a volumetric capacity in an induction system from theexhaust-gas recirculation valve to an inlet of an engine cylinder and avolumetric capacity of the engine cylinder. It is preferable that thecommand exhaust-gas recirculation amount is determined through thepredetermined advance processing which is defined by a first expressionrepresented by Tqec=GKQE#×Tqec0+(GKQE#-1)×Rqec_(n-1), a secondexpression represented by Rqec=Rqec_(n-1) ×(1-Kv)+Tqec0×Kv, and a thirdexpression represented by Kv=Kin×Vc/Vm/CYLN#, where Tqec denotes thecommand exhaust-gas recirculation amount, GKQE# denotes anadvance-processing gain constant, Tqec0 denotes the target exhaust-gasrecirculation amount, Kv denotes a predetermined lag coefficient, Kindenotes a value equivalent to a volumetric efficiency, Vc denotes avolumetric capacity per cylinder, Vm denotes a volumetric capacity ofthe induction system including an intake manifold and a collector, andCYLN# denotes the number of engine cylinders. The command exhaust-gasrecirculation amount setting means may comprise aresponse-characteristic time constant setting means for setting aresponse characteristic between a target exhaust-gas recirculationamount (MQce) which is intended to be drawn into the cylinder and anactual exhaust-gas recirculation amount (Qce) which is actually drawninto the engine cylinder, and a dynamic-characteristic time constantestimation means for estimating dynamic characteristics of arecirculated exhaust gas, flowing through the exhaust-gas recirculationvalve into the engine cylinder, and wherein the command exhaust-gasrecirculation amount which is intended to be passed through theexhaust-gas recirculation valve is set by making the predeterminedadvance processing to the target exhaust-gas recirculation amount (MQce)in accordance with the dynamic characteristics estimated by thedynamic-characteristic time constant estimation means so that theresponse characteristic set by the response-characteristic time constantsetting means is reached. Preferably, the dynamic-characteristic timeconstant estimation means estimates a dynamic-characteristic timeconstant (τa) from an engine speed (Ne), a volumetric efficiency(η_(v)), and a volumetric capacity in an induction system from theexhaust-gas recirculation valve to an inlet of an engine cylinder and avolumetric capacity of the engine cylinder, and whereas theresponse-characteristic time constant setting means sets aresponse-characteristic time constant (τs) at a positive number lessthan the dynamic-characteristic time constant (τa). An integratedinternal combustion engine control system may further comprise anintake-pressure detection means for detecting an intake pressure and anexhaust-pressure detection means for detecting an exhaust pressure, andthe controlled variable setting means calculates a required fluid-flowpassage area (Tav) defined by the exhaust-gas recirculation valve as afunction of the command exhaust-gas recirculation amount (Tqec), theintake pressure (Pm) and the exhaust pressure (Pexh), and sets a targetopening (Mlift) of the exhaust-gas recirculation valve so that thetarget opening (Mlift) is equivalent to the required flow passage area(Tav), and controlled variable setting means including anadvance-processing means through which the controlled variable (Liftt)is set by making a predetermined advance processing to the targetopening (Mlift).

According to another aspect of the invention, an integrated internalcombustion engine control system in combination with an automotiveemission control system, the engine control system comprises anexhaust-gas recirculation valve employed in an exhaust-gas recirculationsystem for sending some of exhaust gas back through an internalcombustion engine, exhaust-gas recirculation amount calculation meansfor calculating a desired exhaust-gas recirculation amount, exhaust-gasrecirculation valve control means for controlling the exhaust-gasrecirculation valve so that the desired exhaust-gas recirculation amountis reached, intake-pressure detection means for detecting an intakepressure, exhaust-pressure detection means for detecting an exhaustpressure, lift detection means for detecting a lift of a valve stem ofthe exhaust-gas recirculation valve, a throttle valve provided in aninduction system for variably throttling intake air drawn into theengine, and throttle valve control means for controlling the throttlevalve depending on the intake pressure, the exhaust pressure, and thelift of the exhaust-gas recirculation valve. An integrated internalcombustion engine control system may further comprise a differentialpressure calculation means for calculating a differential pressure (Dpl)between the exhaust pressure (Pexh) and the intake pressure (Pm), and amaximum recirculated exhaust-gas flow rate calculation means forcalculating a maximum recirculated exhaust-gas flow rate (Qemax) as afunction of the differential pressure (Dpl) and a maximum opening area(Avmax) of the exhaust-gas recirculation valve. The throttle valvecontrol means decreasingly adjusts an opening of the throttle valve whena desired recirculated exhaust-gas flow rate (Tqe) exceeds the maximumrecirculated exhaust-gas flow rate. The throttle valve control meansincreasingly adjusts the opening of the throttle valve when the desiredrecirculated exhaust-gas flow rate (Tqe) is below the maximumrecirculated exhaust-gas flow rate, the differential pressure (Dpl) isabove a predetermined slice level, and/or the lift (Lifts) detected bythe lift detection means is less than a predetermined slice level.

According to a further aspect of the invention, an integrated internalcombustion engine control system in combination with an automotiveemission control system, the engine control system comprises anexhaust-gas recirculation valve employed in an exhaust-gas recirculationsystem for sending some of exhaust gas back through an internalcombustion engine, engine-operating-condition detection means fordetecting an operating condition of the engine, intake-pressuredetection means for detecting an intake pressure, desired exhaust-gasrecirculation rate calculation means for calculating a desiredexhaust-gas recirculation rate or a desired exhaust-gas-recirculationvalve lift (Tlift) based on the operating condition of the engine andthe intake pressure (Pm), and exhaust-gas recirculation valve controlmeans for adjusting an opening of the exhaust-gas recirculation valvetowards the desired exhaust-gas recirculation rate or the desiredexhaust-gas-recirculation valve lift (Tlift).

According to a still further aspect of the invention, an integratedinternal combustion engine control system in combination with anautomotive emission control system, the engine control system comprisesa fuel-injection control system for controlling a fuel-injection amountinjected into a cylinder of an internal combustion engine, thefuel-injection control system including fuel-injection means provided ateach engine cylinder to deliver fuel from a fuel-injection pump,engine-operating-condition detection means for detecting an operatingcondition of the engine, basic fuel-injection amount calculation meansfor calculating a basic fuel-injection amount (Mqdrv) depending on theoperating condition of the engine, first correction means for primarilycorrecting the basic fuel-injection amount (Mqdrv) by at least awater-temperature dependent correction factor to produce a primarilycorrected fuel-injection amount (Drvq), second correction means forsecondarily correcting the primarily corrected fuel-injection amount(Drvq) to produce a secondarily corrected fuel-injection amount (Qsolb)so that a variation of an excess-air factor is kept within an allowablevalue, and fuel-injection amount control means responsive to thesecondarily corrected fuel-injection amount (Qsolb) for controlling afuel-injection amount injected from the fuel-injection means.

According to another aspect of the invention, an intake-air flow ratedetection system for detecting a flow rate of an intake air drawn intoan internal combustion engine, the system comprises air flow ratedetection means for detecting a flow rate of intake air drawn into aninternal combustion engine without distinction of forward flow andreverse flow to output an air flow rate indicative signal (Qo), extremevalue detection means for detecting each of maximal values and minimalvalues from a signal wave of the air flow rate indicative signal,engine-operating-condition detection means for detecting an operatingcondition of the engine, slice-level setting means for setting a slicelevel (Qa2sl) depending on the operating condition of the engine, andreverse-flow decision means for comparing each of the maximal valueswith the slice level and for determining that, when a maximal value inthe signal wave is less than the slice level, a portion ranging withintwo adjacent minimal values in close proximity to the maximal value lessthan the slice level corresponds to a reverse-flow component.

According to a further aspect of the invention, an intake-air flow ratedetection system for detecting a flow rate of an intake air drawn intoan internal combustion engine, the system comprises air flow ratedetection means for detecting a flow rate of intake air drawn into aninternal combustion engine without distinction of forward flow andreverse flow to output an air flow rate indicative signal, count meansfor individually counting a signal-value increasing time interval (C₋₋Inc) and a signal-value decreasing time interval (C₋₋ Dec) with respectto a signal wave of the air flow rate indicative signal, andreverse-flow decision means for making a distinction between forwardflow and reverse flow by comparing the signal-value increasing timeinterval (C₋₋ Inc) with the signal-value decreasing time interval (C₋₋Dec). Preferably, an intake-air flow rate detection system may furthercomprise an advance-processing means for making a predetermined advanceprocessing to the air flow rate indicative signal to produce anadvance-processed signal. The count means individually counts thesignal-value increasing time interval (C₋₋ Inc) and the signal-valuedecreasing time interval (C₋₋ Dec) with respect to a signal wave of theadvance-processed signal. It is preferable that the reverse-flowdecision means makes a distinction between forward flow and reverse flowon the basis of the deviation (DC) between the signal-value decreasingtime interval (C₋₋ Dec) and the signal-value increasing time interval(C₋₋ Inc).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram illustrating a first embodiment of anautomotive emission control system made according to the presentinvention.

FIG. 2 is a flow chart illustrating a routine for calculation of anintake pressure (Pm) of an induction system in a first embodiment of theemission control system.

FIG. 3 is a flow chart illustrating a routine for calculation of anexhaust pressure (Pexh) of an exhaust system in the first embodiment.

FIG. 4 is a flow chart illustrating a main routine for calculation of aninduced fresh-air flow (Qac) per cylinder.

FIG. 5 is a flow chart illustrating a routine for calculation of an EGRamount (Qec) per cylinder.

FIG. 6 is a flow chart illustrating a routine for calculation of aninduced fresh-air temperature (Ta).

FIG. 7 is a flow chart illustrating a routine for calculation of arecirculated exhaust-gas temperature (Te).

FIG. 8 is a flow chart illustrating a routine for calculation of a value(Kin) corresponding to a volumetric efficiency.

FIG. 9 is a graph illustrating the relationship between the previousvalue (Pm_(n-1)) of the intake pressure and an intake-pressure dependentretrieved correction factor (Kinp).

FIG. 10 is a graph illustrating the relationship between an engine speed(Ne) and an engine-speed dependent retrieved correction factor (Kinn).

FIG. 11 is a flow chart illustrating a routine for calculation of anexhaust-gas temperature (Texh).

FIG. 12 is a graph illustrating the relationship between acycle-processed fuel-injection amount (Qfo) and a basic exhaust-gastemperature (Texhb).

FIG. 13 is a flow chart illustrating a routine for calculation of a flowrate (Qe) of EGR.

FIG. 14 is a characteristic curve illustrating the relationship betweenan actual lift (Lifts) of the EGR control valve and an opening area(Ave) of the EGR passage.

FIG. 15 is a flow chart illustrating a routine for a cycle-processingfor each of an induced fresh-air flow (Qac) per cylinder, afuel-injection amount (Qsol), and an intake-air temperature (Tn) of themixture of fresh air (intake air) and exhaust gas delivered from the EGRcontrol valve into the intake manifold.

FIG. 16 is a flow chart illustrating a routine for calculation of acommand lift (Liftt) of the EGR control valve.

FIG. 17 is a characteristic curve illustrating the relationship betweena required flow-passage area (Tav) and a target lift (Mlift).

FIG. 18 is a flow chart illustrating a routine for calculation of arequired flow rate (Tqe) of EGR.

FIG. 19 is a flow chart illustrating a routine for calculation of atarget EGR rate (Megr).

FIG. 20 is a look-up table illustrating the relationship among theengine speed (Ne), the fuel-injection amount (Qsol), and the target EGRrate (Megr).

FIG. 21 is a flow chart illustrating a routine for calculation of thefuel-injection amount (Qsol).

FIG. 22 is a look-up table illustrating the relationship among theengine speed (Ne), a control-lever opening (CL), and a basicfuel-injection amount (Mqdrv).

FIG. 23 is a flow chart illustrating a routine for calculation of thecommand lift (Liftt) of the EGR control valve.

FIG. 24 is a flow chart illustrating a routine for calculation of acommand EGR amount (Tqec).

FIG. 25 is a step-response characteristic curve illustrating simulationresults of the EGR control system included in the emission controlsystem of the first embodiment.

FIG. 26 is a block diagram illustrating a modification of the automotiveemission control system of the invention.

FIG. 27 is a data map used to set the volumetric efficiency (η_(v))based on both the engine speed (Ne) and an induction-collectorinternal-pressure (Pcol).

FIG. 28 is a step-response characteristic curve illustrating simulationresults of the EGR control system included in the emission controlsystem of the modification shown in FIG. 26.

FIG. 29 is a system diagram illustrating a second embodiment of anautomotive emission control system made according to the invention.

FIG. 30 is a block diagram illustrating a control unit employed in theemission control system of the second embodiment.

FIG. 31 is a flow chart illustrating a control routine for theintake-air throttle-valve-opening.

FIG. 32 is a characteristic curve illustrating the relationship betweenan intake-air throttle-valve-opening set parameter Th and the actualintake-air throttle-valve-opening TVO.

FIG. 33 is a flow chart illustrating a routine for calculation of amaximum EGR flow rate (Qemax).

FIG. 34 is a flow chart illustrating another control routine for theintake-air throttle-valve-opening.

FIG. 35 is a flow chart illustrating a routine for calculation of aslice level Liftsl of the lift of the EGR control valve.

FIGS. 36A to 36E are timing charts explaining comparative results of theintake-air throttle-valve-opening control in case of both the improvedsystem (as indicated by the solid line) of the present invention and theprior art system (as indicated by the broken line).

FIG. 37 is a flow chart illustrating an exhaust-gas-recirculationcontrol routine.

FIG. 38 is a flow chart illustrating a routine for calculation of atarget lift (Tlift) of the EGR control valve.

FIG. 39 is a flow chart illustrating another routine for calculation ofa target lift (Tlift) of the EGR control valve.

FIG. 40 is a data map illustrating the relationship among the enginespeed (Ne), a value (Qfe) equivalent to engine load, and the target lift(Tlift) of the EGR control valve.

FIG. 41 is a look-up table illustrating the relationship between thedifferential pressure (dPm) between the actual intake pressure (Pm) andthe target intake pressure (Pmt) and a correction coefficient (Kqf) forthe engine load.

FIG. 42 is a data map illustrating the relationship among the enginespeed (Ne), the engine load, and the target intake pressure (Pmt).

FIGS. 43A to 43F are timing charts explaining comparative results (i.e.,the amount of particulates and the amount of NOx emissions) of the EGRcontrol in case of both the improved system (as indicated by the solidline) of the present invention and the prior art system (as indicated bythe broken line).

FIG. 44 is a flow chart illustrating a main routine for calculation of afuel injection amount Qsol in case of an automotive emission controlsystem of a fourth embodiment.

FIG. 45 is a flow chart illustrating a routine for correction of thefuel-injection amount in the system of the fourth embodiment.

FIG. 46 is a look-up table illustrating the relationship between thedeviation (dEGR) between a target EGR rate (Megr) and the actual EGRrate (Regr) and a correction coefficient (Kqsolh).

FIG. 47 is a flow chart illustrating another routine for correction ofthe fuel-injection amount.

FIG. 48 is a flow chart illustrating another routine for correction ofthe fuel-injection amount.

FIG. 49 is a look-up table illustrating the relationship between theexcess-air-factor equivalent value (Lamb) and an allowable variation(Dlamb) of the excess-air factor.

FIG. 50 is a flow chart illustrating a routine for calculation of themaximum fuel-injection amount (Qful).

FIG. 51 is a look-up table illustrating the relationship between theengine speed (Ne) and an engine-speed dependent retrieved factor(Klambn) related to a limit excess-air factor (Klamb).

FIG. 52 is a look-up table illustrating the relationship between theintake pressure (Pm) and an intake-pressure dependent retrieved factor(Klambp) related to the limit excess-air factor (Klamb).

FIG. 53 is a look-up table used for linearization from an output voltagesignal value (Qo) from the air-flow meter to an intake-air flow rate(Qasm).

FIG. 54 is a flow chart illustrating a main routine for calculation of aweighted average (Qas0) of the intake-air flow rate or an induced-airflow rate in case of the emission control system of a fifth embodiment.

FIG. 55 is a flow chart illustrating a lead processing for theintake-air flow rate.

FIG. 56 is an explanatory view of a difference (a phase lag) between aflow rate detected by the air-flow meter and the actual flow rate drawninto the induction system, due to time constants of the air-flow meter.

FIG. 57 is the former stage of a flow chart illustrating a routine for areverse-flow decision and an intake-air-flow-rate correction.

FIG. 58 is the latter stage of the flow chart shown in FIG. 62.

FIG. 59 is a flow chart illustrating a routine for derivation of acomparative value or a slice level (Qa2sl).

FIG. 60 is a look-up table illustrating the relationship between theengine speed (Ne) and the slide level (Qa2sl).

FIG. 61 is a flow chart illustrating another routine for derivation of aslice level (Qa2sl).

FIG. 62 is a characteristic curve illustrating the relationship betweenthe intake-air throttle-valve-opening (TVO) and a intake-airthrottle-valve-opening dependent slice-level correction coefficient(Kqa2sl).

FIG. 63 is a flow chart illustrating an averaging routine for thereverse-flow corrected intake-air flow rates (Qas03).

FIG. 64 is an explanatory view illustrating the reverse-flow correction.

FIGS. 65A-65F are timing charts illustrating various signal waveformsobtained through an arithmetic operation of the system of the fifthembodiment.

FIG. 66 is a graph illustrating comparative results among the actualintake-air flow rate, the intake-air flow rate (indicated by the solidline) obtained through the arithmetic operation of the system of thefifth embodiment and the intake-air flow rate (indicated by theone-dotted line) obtained through the arithmetic operation of the priorart system.

FIG. 67 is a flow chart illustrating another routine for decision of anextreme value of the voltage signal from the air-flow meter.

FIG. 68 is a flow chart illustrating a sub-routine for counting both anincreasing time-interval and a decreasing time-interval of the voltagesignal from the air-flow meter.

FIG. 69 is a simplified timing chart explaining the relationship betweena base signal indicative of simplified pulsation flow, a signal-valuedecreasing time-interval indicative signal C₋₋ Dec, a signal-valueincreasing time-interval indicative signal C₋₋ Inc, a deviation signalDC, and a converted signal (including an inversed signal based on thereverse-flow decision).

FIG. 70 is a flow chart illustrating a routine for an inversionprocedure based on the reverse-flow decision.

FIGS. 71A to 71C are timing charts respectively illustrating a waveformof a signal from the hot-wire type air-flow meter, a waveform of asignal obtained through the advance-processing, and a waveform of asignal obtained through the reverse-flow correction.

FIG. 72 is a graph illustrating simulation results representative of therelation among an actual intake-air flow rate, a waveform of the outputsignal from the air-flow meter, a waveform of an intake-air flow rateindicative signal properly corrected through the system of the fifthembodiment, and a waveform of an intake-air flow rate indicative signalobtained through the prior art system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

Referring now to the drawings, particularly to FIGS. 1 through 25, theautomotive emission control system of the invention is exemplified incase of a diesel engine with a turbocharger. As seen in FIG. 1,reference numeral 1 denotes a typical turbocharger with a compressorpump 1A and a turbine 1B. In a conventional manner, the turbine wheel isspun by the exhaust gas. The turbine wheel is the same shaft as thecompressor-pump rotor, and so the compressor pump is driven insynchronization with rotation of the turbine wheel so as to produce ahigh pressure on fresh air which is introduced through an air filter 2into an air intake passage or an induction passage 3. The super-chargedair is directed into an intake manifold 4. The engine 5 is equipped withfuel-injection nozzles 6 at each cylinder to deliver fuel from afuel-injection pump 7 directly into combustion chambers of the enginecylinders near the top dead center (T. D. C.) position on thecompression stroke. The emission control system of the embodimentincludes an exhaust-gas recirculation (abbreviated simply as "EGR")passage 10 which connects the exhaust manifold 8 with the intakemanifold 4, and an EGR control valve 9 (EGR valve) fluidly disposed inthe EGR passage 10. The EGR passage 10 and the EGR valve 9 are providedto send some of the exhaust gas back through the engine to the intakemanifold 4, thereby reducing the formation of nitrogen oxides (NOx).Disposed in the induction passage 3 upstream of the compressor pump 1Ais an intake-air throttle valve 31, simply called a throttle valve, soas to constrict the fresh-air flow during the EGR control andconsequently to enlarge the differential pressure between the exhaustpressure and the intake pressure, for the purpose of facilitatingrecirculation of the exhaust gas. The EGR control is effective to reduceexhaust emissions and exhaust noise particularly during the engineidling or during the low engine load. The EGR control is actuallyperformed by decreasingly adjusting the opening of the throttle valve 31and simultaneously by properly adjusting the opening of the EGR valve 9.As seen in FIG. 1, the throttle valve 31 is usually comprised of abutterfly valve. The butterfly-type throttle valve 31 is linked to avacuum-operated mechanism which consists of a diaphragm unit 33 and anelectromagnetic valve 32, so that the angular position (or the opening)of the throttle valve 31 is adjusted by way of the vacuum fed into thediaphragm chamber of the diaphragm unit 33 through the valve 32. Thevacuum is produced by a vacuum pump 11, which is located near the enginecylinder block, and delivered through a vacuum line (a vacuum tube) tothe inlet port of the valve 32. Although a signal line for theelectromagnetic valve 32 is not shown in FIG. 1, the valve 32 can beopened or closed depending on a control signal from a control unit 13,in order to open or close the vacuum line. With the valve 32 fullyopened, the vacuum raises the diaphragm (see reference sign 52 of FIG.29 showing the detail of the EGR valve) of the unit 33 and as a resultthe opening of the throttle valve 31 decreases and thus the fresh airflow is constricted properly. On the other hand, the EGR valve 9 has asignal port or a pilot pressure port (corresponding to reference sign 54of FIG. 29) connected to an outlet port of a duty-cycle controlledelectromagnetic solenoid valve 12, to introduce a pressure that isproperly adjusted by the valve 12 into the diaphragm chamber (seereference sign 55 of FIG. 29) of the EGR valve. Although it is notclearly shown in the drawing, the duty-cycle controlled valve 12 has anatmospheric-pressure inlet port, such that the atmospheric-pressureinlet port (an air bleed) is cyclically opened and closed according to aduty cycle or a duty factor which is determined by the control unit 13.In other words, the duty-cycle controlled valve 12 is provided tosuitably dilute the incoming vacuum with the atmosphere. So, air of thepartial vacuum (the negative pressure) in the pressure chamber of thevalve 12 is properly mixed with the atmosphere of a normal atmosphericpressure, and thus the outgoing vacuum (the negative gauge pressure)from the valve 12 can be properly adjusted at a higher pressure levelthan the incoming vacuum. The lower a reading of vacuum in the diaphragmchamber of the EGR valve 9, the greater the lift of the valve 9. Thatis, when more of the vacuum fed into the valve 12 is delivered into theEGR valve 9 through the signal line, the vacuum raises the diaphragm ofthe EGR valve 9 almost to the uppermost position at which the lift ofthe valve 9 reaches approximately the maximum amount, because of the EGRvalve shaft firmly connected to the diaphragm. In such a case, the EGRvalve 9 may produce a substantially maximum exhaust-gas-recirculationrate (EGR rate). In this manner, the lift of the EGR valve 9 can beproperly adjusted and whereby the EGR rate can be varied properlydepending on the duty factor determined by the control unit 13. As seenin FIG. 1, a lift sensor 34 may be located at the EGR valve 9 fordirectly sensing the actual lift (Lifts) of the EGR valve 9. After thecombustion stroke (or the power stroke), burnt gases are forced from theindividual cylinder to the exhaust manifold 8 and then the exhaust gasflow rotates the exhaust turbine 1B. Thereafter, the exhaust gases areexhausted through a filter 14 and a muffler 15 into the atmosphere. Thefilter 14 is provided to remove particulates and smoke included in theexhaust gases, whereas the muffler 15 is provided to reduce the exhaustnoise. An air-flow meter 16 is disposed in the induction passage 3upstream of the air compressor 1A, for detecting a flow rate Qo of thefresh air passing through the air cleaner 2 which is provided forfiltering dust and dirt out of the fresh air drawn into the engine. Alsoprovided are various sensors, namely an engine speed sensor 17 (forsensing the engine speed Ne), a water-temperature sensor 18 (for sensingthe water temperature Tw), and a lever-opening sensor 19 (for sensing anopening CL of the control lever of the fuel-injection pump 7). As willbe hereinafter described in detail, a pressure in the induction system(including the intake manifold and the collector), simply abbreviated"intake pressure", and a pressure in the exhaust system, simplyabbreviated "exhaust pressure" are derived or estimated on the basis ofthe signals from the respective sensors 16, 17, 18 and 19 by means ofthe control unit 13, so as to properly set the maximum permissiblefuel-injection amount. Alternatively, the above-mentioned intakepressure may be sensed by means of an intake-pressure sensor 35 which isattached to the intake manifold 4 downstream of the air compressor 1A.The control unit 13 employed in the system of the first embodimentoperates as follows.

Referring now to FIG. 2, there is shown a routine for calculation of theintake pressure Pm. In step S1, read are an induced fresh-air flow percylinder Qac, an EGR amount per cylinder Qec, an induced fresh-airtemperature Ta, a recirculated exhaust-gas temperature Te which will behereinafter referred to simply as an "EGR temperature", and a value Kincorresponding to a volumetric efficiency which will be hereinafterreferred to as a "volumetric-efficiency equivalent value". As discussedlater, these parameters Qac, Qec, Ta, Te, and Kin are determined throughanother arithmetic-operation routines. In step S2, the intake pressurePm is calculated on the basis of a predetermined volumetric ratio(Vc/Vm) of the volumetric capacity/cylinder (Vc) with respect to thecollector and intake-manifold volumetric capacity Vm in the inductionsystem, according to the following expression, for example. ##EQU1##where Kvol is equal to the volumetric ratio (Vc/Vm), KPM is apredetermined constant.

Referring to FIG. 3, there is shown a routine for calculation of theexhaust pressure Pexh. In step S11, read are a displacement per cylinderQexh which is exhausted from one cylinder, the EGR amount per cylinderQec, an exhaust-gas temperature Texh, and the engine speed Ne. In stepS12, on the basis of the above parameters Qexh, Qec, Texh, and Ne, theexhaust pressure Pexh is calculated from the following expression.

    Pexh=(Qexh-Qec)×Texh×Ne×Kpexh+Opexh

where the values Kpexh and Opexh are predetermined constants.

The previously-noted induced fresh-air flow per cylinder Qac iscalculated in accordance with the routine indicated by the flow chart ofFIG. 4.

In step S21, read is an output signal value Qo (in the form of a voltagesignal) generated from the air-flow meter 16.

In step S22, the voltage signal value Qo is converted into an intake-airflow rate (an induced fresh-air flow rate) Qasm through linearizationaccording to a predetermined conversion table or a linearization table(see FIG. 53).

In step S23, a weighted mean processing is executed to derive a weightedmean Qas0 from the intake-air flow rate Qasm.

In step S24, read is a value of an engine-speed indicative signal Nefrom the engine speed sensor 17.

In step S25, an intake-air (induced-air) flow per cylinder Qac0 iscalculated on the basis of the weighted mean Qas0 of the inducedfresh-air flow rate and the engine-speed indicative signal value Ne,according to the following expression.

    Qac0=Qas0/Ne×KCON#

where KCON# is a predetermined constant.

In step S26, a so-called delay processing is executed, since the freshair having the induced-air flow rate detected instantaneously by theair-flow meter 16 is fed into the induction-collector inlet with acertain lag time. Usually, the routine shown in FIG. 4 is executed astime-triggered interrupt routines to be triggered every predeterminedtime intervals. Actually, n data of the induced fresh-air flow percylinder, namely Qac0(1), Qac0(2), Qac0(3), . . . , Qac0(n-2), Qac0(n-1)and Qac0(n) are stored in predetermined memory addresses of the controlunit 13. The data Qac0(1) represents a fresh-air flow per cylinderderived through the arithmetic operation (see the flow from step S21 tostep S25) of FIG. 4 before n cycles, whereas the data Qac0(n) representsa fresh-air flow per cylinder derived at the current cycle. Inconsideration of the cycle delay (or the phase delay), the data Qac0(1)is regarded and read as a fresh-air flow Qac_(n) currently drawn intothe induction-collector inlet. Thus, Qac_(n) means the current value ofthe induced fresh-air flow drawn into the collector inlet, whileQac_(n-1) means the previous value of the induced fresh-air flow. Thecurrent value Qac_(n) of the induced fresh-air flow is represented asthe expression Qac_(n) =Qac0·Z^(-n).

In step S27, a final induced fresh-air flow per cylinder Qac isestimated and derived from the volumetric ratio Kvol (=Vc/Vm) and thevolumetric-efficiency equivalent value Kin according to the followingexpression which corresponds to an expression of first-order lag.

    Qac=Qac.sub.n-1 ×(1-Kvol×Kin)+Qacn×Kvol×Kin

where the product (Kvol×Kin) of the volumetric ratio Kvol and thevolumetric-efficiency equivalent value Kin means what percent of thefresh air currently induced into the induction collector can be actuallydrawn into the cylinder. Therefore, owing to the first-order lag, thefirst term {Qac_(n-1) ×(1-Kvol×Kin)} corresponds essentially to the rateof fresh air flow which will be drawn into the cylinder from within theinduced fresh-air flow measured by the air-flow meter at the previousarithmetic-operation cycle (see FIG. 4), while the second term(Qacn×Kvol×Kin) corresponds essentially to the rate of fresh air flowwhich will be drawn into the cylinder from within the induced fresh-airflow measured by the air-flow meter at the current arithmetic-operationcycle.

As may be appreciated from the above, the induced fresh-air flow ratecan be estimated accurately.

During the EGR control, the EGR amount per cylinder Qec is calculated inaccordance with the routine shown in FIG. 5.

In step S31, read is the flow rate Qe of exhaust gas recirculated intothe induction system (into the intake manifold 4). The flow rate Qe ofrecirculated exhaust gas will be hereinafter referred to simply as an"EGR flow rate". As will be discussed later, the EGR flow rate Qe can bederived through another sub-routine.

In step S32, the engine speed Ne is read.

In step S33, an EGR amount per cylinder Qecn is calculated on the basisof the EGR flow rate Qe, the engine speed Ne and the predeterminedconstant KCON# according to the following expression.

    Qec.sub.n =Qe/Ne×KCON#

In step S34 similar to step S27, a final EGR amount per cylinder Qec isestimated and derived from the volumetric ratio Kvol (=Vc/Vm) and thevolumetric-efficiency equivalent value Kin according to the followingexpression.

    Qec=Qec.sub.n-1 ×(1-Kvol×Kin)+Qec.sub.n ×Kvol×Kin

where the product (Kvol×Kin) means what percent of the EGR amount percylinder currently calculated is actually drawn into the cylinder.Therefore, the first term {Qec_(n-1) ×(1-Kvol×Kin)} correspondsessentially to the rate of the EGR amount per cylinder which will bedrawn into the cylinder from within the EGR amount per cylinderQec_(n-1) calculated at the previous arithmetic-operation cycle (seeFIG. 5), while the second term (Qec_(n) ×Kvol×Kin) correspondsessentially to the rate of the EGR amount per cylinder which will bedrawn into the cylinder from within the EGR amount per cylinder Qec_(n)calculated at the current arithmetic-operation cycle.

Referring now to FIG. 6, the induced fresh-air temperature Ta can bederived from the previous value Pm_(n-1) of the intake pressure. Thatis, in step S41, the previous value Pm_(n-1) of the intake pressure isread. Thereafter, in step S42, the fresh-air temperature Ta is derivedfrom the following expression, based on the well-known law ofthermodynamics (an adiabatic law).

    Ta=TA#×(Pm.sub.n-1 /PA#).sup.(K- 1)/K+TOFF#

where TA# and PA# respectively denote a predetermined standardtemperature (a predetermined constant) and a predetermined standardpressure (a predetermined constant), both being constants, K denotes aratio of specific heats, and TOFF# denotes a temperature rise occurringdue to a pressure rise in the intake pressure risen while the fresh airis drawn through the air cleaner into the induction collector. To moreprecisely estimate the fresh-air temperature Ta, the standardtemperature TA# and the temperature rise TOFF# may be corrected bymultiplying them by respective correction factors KtA and KtOFF, whichare usually determined to be proportional to a rise in the watertemperature Tw.

Referring now to FIG. 7, there is shown a routine for calculation of thetemperature Te of the exhaust gas recirculated into theinduction-collector inlet. In step S51, read is the exhaust-gastemperature Texh which is derived by another sub-routine as will beexplained later. In step S52, the EGR temperature Te is calculated inaccordance with the following expression.

    Te=Texh×KTLOS#

where KTLOS# denotes a temperature-drop factor correlated to a rate oftemperature drop of the recirculated exhaust gas flowing from theexhaust manifold to the intake manifold.

Referring now to FIG. 8, there is shown a routine for calculation of thevolumetric-efficiency equivalent value Kin. In step S61, read are theprevious value Pm_(n-1) of the intake pressure and the engine speed Ne.In step S62, the intake-pressure dependent retrieved correction factorKinp is derived from the previous value Pm_(n-1) of the intake pressurein accordance with the look-up table as shown in FIG. 9. In step S63,the engine-speed dependent retrieved correction factor Kinn is derivedfrom the engine speed Ne in accordance with the look-up table as shownin FIG. 10. In step S64, the volumetric-efficiency equivalent value Kinis calculated or estimated as the product (Kinp×Kinn) of both theintake-pressure dependent retrieved correction factor Kinp and theengine-speed dependent retrieved correction factor Kinn. If the enginesystem employs a swirl control valve, the volumetric-efficiencyequivalent value Kin may be corrected by a swirl-control-valve-openingdependent correction factor Kins which is usually determined inproportion to the opening of the swirl control valve. In this case, thevolumetric-efficiency equivalent value Kin is expressed as the followingequation.

    Kin=Kinp×Kinn×Kins

Referring now to FIG. 11, there is shown a routine for calculation ofthe exhaust-gas temperature Texh. As may be appreciated, this arithmeticoperation of FIG. 11 is unnecessary in case that an exhaust-gastemperature sensor is provided for directly sensing the temperature ofthe exhaust gas. For the purpose of calculating the exhaust-gastemperature Texh, two data Qfo and Tno, both obtained through aso-called cycle processing shown in FIG. 15, are used. As will behereinafter detailed, the cycle-processing shown in FIG. 15 is similarto the delay-processing previously explained in step S26 of the flowchart shown in FIG. 4.

In step S71, a so-called cycle-processed fuel-injection amount Qfo isread. The cycle-processed fuel-injection amount Qfo can be derived inconsideration of the cycle delay (the phase delay) from the time whenthe nozzle injects fuel on the intake stroke up to the exhaust stroke,in accordance with step S92 of FIG. 15.

In step S72, a so-called cycle-processed intake-air temperature Tno isread. Similarly to step S71, the cycle-processed intake-air temperatureTno can be derived in consideration of the cycle delay, according tostep S92 of FIG. 15.

In step S73, read is the previous value Pexh_(n-1) of the exhaustpressure which is calculated one cycle before according to thearithmetic operation of FIG. 3.

In step S74, a basic exhaust-gas temperature Texhb is derived orretrieved from the above-noted cycle-processed fuel-injection amount Qfoby reference to the look-up table as shown in FIG. 12.

In step S75, an intake-air temperature dependent correction factorKtexh1 is derived from the cycle-processed intake-air temperature Tno bythe following expression.

    Ktexh1=(Tno/TA#).sup.KN

where TA# denotes the standard temperature previously discussed, and KNdenotes an exponent of a ratio (Tno/TA#) of the cycle-processedintake-air temperature Tno to the standard temperature TA# and preset ata predetermined constant. The intake-air temperature dependentcorrection factor Ktexh1 corresponds essentially to a rate of theexhaust-gas temperature-rise occurring due to the rise in the intake-airtemperature.

In step S76, an exhaust-pressure dependent correction factor Ktexh2 isderived from the previous value Pexh_(n-1) of the exhaust pressure bythe following expression based on an well-known adiabatic law (anadiabatic change).

    Ktexh2=(Pexh.sub.n-1 /PA#).sup.(Ke- 1)/Ke

where PA# denotes the standard pressure previously discussed, (Ke-1)/Kedenotes an exponent of a ratio (Pexh_(n-1) /PA#), and Ke is preset at apredetermined constant. The exhaust-pressure dependent correction factorKtexh2 corresponds essentially to a rate of the exhaust-gastemperature-rise occurring due to the rise in the exhaust pressure.

In step S77, the exhaust-gas temperature Texh is derived from the basicexhaust-gas temperature Texhb, the two correction factors Ktexh1 andKtexh2, according to the following expression.

    Texh=Texhb×Ktexh1×Ktexh2

Referring to FIG. 13, there is shown a routine for calculation of theEGR flow rate Qe. In step S81, are read the intake pressure Pm, theexhaust pressure Pexh, an actual lift (Lifts) of the EGR valve 9, andthe EGR temperature Te. The actual lift Lifts is sensed by the liftsensor 34 and the sensed lift indicative signal is transferred into theinput interface of the control unit 13.

In step S82, an opening area (Ave) of the EGR passage or the EGR valveis retrieved from the actual lift Lifts of the EGR valve in accordancewith the look-up table as shown in FIG. 14. In step S83, the EGR flowrate Qe is calculated on the basis of the four parameters Pm, Pexh,Lifts (or Ave), and Te, in accordance with the following expression.

    Qe=Ave×(Pexh-Pm).sup.1/2 ×KR#/Te×TA#

where KR# is a predetermined constant. As is generally known, afluid-flow velocity q is expressed as q=(ΔP-·2ρp)^(1/2), where ΔPdenotes a differential pressure for example between a pressure in theinlet of the EGR passage and a pressure in the outlet of the EGRpassage, and ρ denotes a mass density of the recirculated exhaust gasflow. The above-mentioned predetermined constant KR# is selected to beessentially equivalent to the value (2ρ)^(1/2).

In the previously-discussed expression, although the four parameters Pm,Pexh, Lifts (or Ave), and Te are used for deriving the EGR flow rate Qe,the parameter Te is often omitted. That is, the EGR flow rate Qe may beexpressed simply as Qe=Ave×(Pexh-Pm)^(1/2) ×KR#, because the EGR flowrate Qe is slightly affected by the rise in the EGR temperature.

Referring to FIG. 15, there is shown the cycle-processing similar to thedelay-processing (see step S26 of FIG. 4). In step S91, first of all,read are the induced fresh-air flow per cylinder Qac, the fuel-injectionamount Qsol and the intake-air temperature Tn. In the shown embodiment,the intake-air temperature Tn is calculated in accordance with theexpression Tn=(Qac×Ta+Qec×Te)/(Qac+Qec), where Qac denotes the inducedfresh-air flow per cylinder, Ta denotes the fresh-air temperature, Qecdenotes the EGR amount per cylinder, and Te denotes the EGR temperature.

In step S92, the cycle-processing is executed as follows.

As per the induced fresh-air flow per cylinder Qac related to the intakestroke, for the purpose of phase matching (or cycle matching) to theexhaust stroke, the previously-noted delay-processing is executed sothat the number (CYLN#-1), obtained by subtracting "1" from the number(CYLN#) of engine cylinders, is selected as a value equivalent to thephase delay of the induced fresh-air flow per cylinder Qac derivedthrough the arithmetic-operation routine of FIG. 4. That is, the dataQac·Z⁻(CYLN#-1) which is derived through the arithmetic operation (seethe flow of step S21 to step S27) of FIG. 4 before (CYLN#-1) cycles, isregarded as the displacement per cylinder Qexh exhausted from onecylinder during the exhaust stroke.

As per the fuel-injection amount Qsol related to the compression stroke,for the purpose of phase matching to the exhaust stroke, thepreviously-noted delay-processing is executed so that the number(CYLN#-2), obtained by subtracting "2" from the number (CYLN#) of enginecylinders, is selected as a value equivalent to the phase delay of thefuel-injection amount Qsol which amount is derived through the routineof FIG. 21 as will be explained later. That is, the dataQsol·Z⁻(CYLN#-2) which is derived through the arithmetic operation (seethe flow of step S131 to step S134) of FIG. 21 before (CYLN#-2) cycles,is regarded as the cycle-processed fuel-injection amount Qfo.

As per the intake-air temperature Tn related to the intake stroke, forthe purpose of phase matching to the exhaust stroke, thepreviously-noted delay-processing is executed so that the number(CYLN#-1), obtained by subtracting "1" from the number (CYLN#) of enginecylinders, is selected as a value equivalent to the phase delay of theintake-air temperature Tn derived through step 91 of FIG. 15. That is,the data Tn·Z⁻(CYLN#-1) which is derived through the arithmeticoperation of FIG. 15 before (CYLN#-1) cycles, is regarded as thecycle-processed intake-air temperature Tno.

Referring now to FIGS. 16 to 18, there is shown an EGR control executedby the system of the first embodiment. A command lift Liftt of the EGRvalve 9 is calculated according to the routine shown in FIG. 16. In stepS101, read are the intake pressure Pm, the exhaust pressure Pexh, arequired EGR flow rate Tqe, and the EGR temperature Te. In step S102, arequired fluid-flow passage area Tav defined by the EGR valve 9 iscalculated as a function of the four parameters Pm, Pexh, Tqe and Te, asfollows.

    Tav=Tqe/(Pexh-Pm).sup.1/2 /KR#×Te/TA#

where KR# and TA# are predetermined constants as previously discussed.As already explained in step S83 of FIG. 13, the parameter Te may beomitted. In this case, the required fluid-flow passage Tav is expressedsimply as Tav=Tqe/(Pexh Pm)^(1/2) /KR#.

In step S103, a target lift Mlift of the EGR valve is derived orretrieved from the required fluid-flow passage Tav in accordance withthe look-up table as shown in FIG. 17.

In step S104, a so-called advance processing or lead processing (as willbe hereinbelow explained in detail with respect to the sub-routine shownin FIG. 23) is made to the target lift Mlift in consideration of thedelay in actuating timing of the EGR valve. The. advance-processedtarget lift Mlift is regarded as the command lift Liftt, and then acontrol signal equivalent to the command lift Liftt is output from thecontrol unit 13 to the duty-cycle controlled electromagnetic valve 12.

Referring to FIG. 18, there is shown a routine for calculation of therequired EGR flow rate Tqe. In step S111, read are the engine speed Ne,a target EGR rate Megr, and the induced fresh-air flow per cylinder Qac.In step S 112, a target EGR amount Tqec0 is derived as the product(Qac×Megr) of the induced fresh-air flow per cylinder Qac and the targetEGR rate Megr. In step S113, to derive a command EGR amount Tqec, aso-called advance processing is made to the target EGR amount Tqec0derived in step S112, in consideration of the volumetric capacity in theinduction system from the EGR control valve to the inlet of the enginecylinder and the volumetric capacity of the cylinder. The detail of theadvance processing will be discussed later with respect to the flowchart of FIG. 24. In step S114, the required GR flow rate Tqe is derivedas a function of the command EGR amount Tqec and the engine speed Ne,according to the following expression.

    Tqe=Tqec×Ne/KCON#

where KCON# is the same predetermined constant as discussed in step S25of FIG. 4 and in step S33 of FIG. 5.

Referring to FIG. 19, there is shown a routine for calculation of theEGR rate Megr. In step S121, read are the engine speed Ne and thefuel-injection amount Qsol. In step S122, the target EGR rate Megr isretrieved from both the engine speed Ne and the fuel-injection amountQsol, which injection amount is substantially representative of theengine load, by reference to the look-up table as shown in FIG. 20.

Referring to FIG. 21, there is shown a routine for calculation of thefuel-injection amount Qsol. In step S131, read are the engine speed Neand the control-lever opening CL of the injection pump 7. In step S132,a basic fuel-injection amount Mqdrv is retrieved from both the enginespeed Ne and the control-lever opening CL, according to the look-uptable as shown in FIG. 22. In step S133, the basic fuel-injection amountMqdrv is corrected by various correction factors such as awater-temperature dependent correction factor and the like, to produce acorrected fuel-injection amount Qsol1. In step S134, in the event thatthe corrected fuel-injection amount Qsol1 exceeds an upper limit (agiven maximum fuel-injection amount Qful as calculated through anothersub-routine shown in FIG. 50), the corrected fuel-injection amount Qsol1is replaced with the upper limit to keep the actual output value of thefuel-injection amount Qsol within the upper limit. When the correctedfuel-injection amount Qsol1 is below the upper limit, the correctedfuel-injection amount Qsol1 is regarded as the actual output value ofthe injection amount Qsol.

FIG. 23 shows the advance processing (see step S104 of FIG. 16)necessary to derive the command lift Liftt for the EGR valve from thetarget lift Mlift. In step S145, read is the target lift Mlift derivedthrough step S103. In step S146, a test is made to determine whether thecurrent value Mliftn of the target lift Mlift is greater than or equalto the previous value Mliftn-1 of the target lift. The current valueMliftn of the target lift will be hereinafter abbreviated simply as"Mlift". When the answer to step S146 is affirmative (YES), step S147proceeds in which a time constant Tcl is set at a first predeterminedtime constant TCL1# corresponding to a time constant or a lagcoefficient in case that the valve lift of the EGR valve 9 increases.When the answer to step S146 is negative (NO), step S148 proceeds inwhich the time constant Tcl is set at a second predetermined timeconstant TCL2# corresponding to a time constant or a lag coefficient incase that the valve lift decreases. In step S149, on the basis of thecurrent target lift Mlift and the time constant Tcl obtained throughstep S147 or S148, a time-constant dependent function Rlift is expressedas Rliftn=Rlift_(n-1) ×(1-Tcl)+Mlift×Tcl, where Rliftn denotes thecurrent value of the function Rlift, Rlift_(n-1) denotes the previousvalue of the function Rlift, and Tcl is the selected time constant. Thecommand lift Liftt is derived from both the current target lift Mliftand the previous value Rlift_(n-1) of the function Rlift, according tothe following expression.

    Liftt=GKL#×Mlift-(GKL#-1)×Rlift.sub.n-1

where GKL# is an advance-processing gain (a predetermined constant).

Referring to FIG. 24, there is shown the advance processing (see stepS113 of FIG. 18) necessary to derive the command EGR amount Tqec. Instep S151, read is the target EGR amount Tqec0 (=Qac×Megr) derivedthrough step S112 of FIG. 18. In step S152, on the basis of the currenttarget EGR amount Tqec0 and a predetermined correction factor (aconstant) Kv, a volumetric-efficiency dependent function Rqec isexpressed as Rqec (=Rqec_(n))=Rqec_(n-1) ×(1-Kv)+Tqec0×Kv, whereRqec_(n) denotes the current value of the function Rqec, Rqec_(n-1)denotes the previous value of the function Rqec, and the correctionfactor Kv is expressed as Kv=Kin×Vc/Vm/CYLN# (=Kin×Kvol/CYLN#). In thiscase, the product (Kin×Kvol) represents a percentage of the EGR amountwhich will be actually drawn into the cylinder. That is to say, thecorrection factor corresponds to a predetermined lag coefficient. Thecharacter CYLN# denotes the number of engine cylinders. The command EGRamount Tqec is derived from both the current target EGR amount Tqec0 andthe previous value Rqec_(n-1) of the function Rqec, according to thefollowing expression.

    Tqec=GKQE#×Tqec0+(GKQE#-1)×Rqec.sub.n-1

where GKQE# means an advance-processing gain constant (a predeterminedconstant).

As will be appreciated from the above, the induced fresh-air flow percylinder Qac is precisely estimated in consideration of the first-orderlag on the basis of the fresh-air flow rate indicative signal Qo fromthe air-flow meter 16, and the target EGR amount Tqec0 is calculated asa function of the induced fresh-air flow per cylinder Qac and the targetEGR rate Megr based on both the engine speed Ne and the fuel-injectionamount Qsol, and in consideration of both the volumetric capacity in theinduction system from the EGR valve to the inlet of the cylinder and thevolumetric capacity of the cylinder the advance-processed target EGRamount Tqec0 is updated as the command EGR amount Tqec. Additionally,the required fluid-flow area Tav for the EGR passage or the EGR valve isproperly determined depending on the differential pressure (Pexh-Pm)between the exhaust pressure and the intake pressure as well as therequired EGR flow rate Tqe, and the target lift Mlift of the EGR valveis derived from the required fluid-flow area, and thereafter theadvance-processing is made to the target lift Mlift in consideration ofthe delay in actuating timing of the EGR valve, and finally theadvance-processed target lift Mlift is updated as the command lift Liftt(the actual duty-cycle signal value to be output to the valve 12) forthe EGR valve. That is, the target EGR amount is determined depending onthe induced fresh-air flow rate as well as the target EGR rate. Theinduced fresh-air flow rate (the flow rate per cylinder Qac) based onthe measured value of the air-flow meter 16 varies in response to thechange in the density of the induced fresh air, while the EGR rate Megrbased on the engine speed Ne and the engine load (the fuel-injectionamount Qsol) is not influenced by the change in air density. Thus, thetarget EGR amount can be effectively optimized in response to theenvironmental change (the change in air density). Although there is aresponse delay (a phase delay between the recirculated exhaust-gas flowpassing through the EGR valve and the recirculated exhaust-gas flowactually drawn into the inlet of the engine cylinder) owing to a dynamiccharacteristic of the recirculated exhaust gas from the EGR valve to thecylinder inlet particularly in a transition state such as during hardacceleration, a command EGR amount is derived from the target EGR amountthrough an advance processing reflecting the dynamic characteristic andthus the system prevents the response delay from producing a badinfluence on the EGR control. The time constant often called lagcoefficient is determined depending on the volumetric capacity in theinduction system from the EGR control valve to the engine-cylinder inletand the volumetric capacity of the cylinder, and thus the advanceprocessing effectively compensates the phase delay. Accordingly, thesystem of the first embodiment can provide a high-precision EGR controlas previously explained and thus ensure an optimal exhaust emissioncontrol. So, harmful exhaust emissions such as NOx emissions,particulates, and the like, are effectively reduced by virtue of theoptimal EGR control of the system of the first embodiment, even in caseof the transition state of the vehicle driving such as in a transitionfrom a constant-speed driving to a hard acceleration, or in the presenceof a remarkable change in air density, occurring due to theenvironmental change from low-land driving to high-land driving.

Briefly speaking, according to the system of the first embodiment,firstly a first target EGR amount which is intended to be drawn into thecylinder is set, and secondly the time constant of the dynamiccharacteristics of the recirculated exhaust gas, flowing from the EGRvalve through the EGR passage to the engine cylinder, is estimated onthe basis of the estimated volumetric efficiency (thevolumetric-efficiency equivalent value Kin) based on at least the enginespeed Ne. Thirdly, a second target EGR amount which is intended to bepassed through the EGR valve is arithmetically derived through a firstadvance processing reflecting the estimated time constant. Thereafter,the target lift Mlift for the EGR valve is estimated based on the secondtarget EGR amount. In the first embodiment, a second advance processingreflecting the delay in actuating timing of the EGR valve is made to thetarget lift Mlift to derive the command lift Liftt. As regards the firstadvance processing, assuming that the time constant of the dynamiccharacteristics of the recirculated exhaust-gas flow is represented asra, the second target EGR amount M₂ Qe is expressed as the followingLaplace-transformation operation expression (1).

    M.sub.2 Qe={(1+G·τa·s)/(1+τa·s)}·MQce(1)

where M₂ Qe (kg/stroke) denotes the second target EGR amount which isintended to be passed through the EGR valve, G denotes a predeterminedadvance-processing gain, τa (sec) denotes the time constant indicativeof dynamic characteristics of the recirculated exhaust gas, flowing fromthe EGR valve through the EGR passage to the engine cylinder, s denotesa Laplace operator (Laplacian), and MQce (kg/stroke) denotes the firsttarget EGR amount which is intended to be drawn into the enginecylinder. As a result of the above Laplace-transformation operationexpression (1), an actual EGR amount Qce which will be actually drawninto the cylinder, is expressed as the following approximate expression(2) on the assumption that the above-noted second target EGR amount M₂Qe is equal to the EGR amount actually passing through the EGR valve.

    Qce={(1+G·τa)/(1+τa).sup.2 }·MQce(2)

As may be appreciated from the expressions (1) and (2), there is apossibility that the actual EGR amount Qce overshoots the first targetEGR amount MQce, depending on the magnitude of the advance-processinggain G. Supposing the excessively less gain G is selected in order toreduce such an overshoot, the responsibility of the EGR control may bedeteriorated. FIG. 25 shows simulation results of the actual EGR amountQce when a so-called step input is applied as the first target EGRamount MQce in a conventional step-response method, in the presence ofthe advance-processing of the expression (1) at various gains such as 2,1.5 and 0.8 and in the absence of the processing of the expression (1).As can be appreciated from the simulation results of FIG. 25, in thepresence of the processing of the expression (1), there is a tendencyfor the overshoot to occur at a gain G (for example G=2, G=1.5) above"1", while there is a tendency for the responsibility of the EGR controlto deteriorate at a gain (for example G=0.8) below "1". To eliminate theabove-noted points, FIG. 26 shows a further improved system (amodification) of the first embodiment.

Referring now to FIG. 26, there is shown the system diagram of thefurther improved EGR control system. The system of FIG. 26 includes atarget EGR amount per cylinder setting section 41, aresponse-characteristic time constant τs setting section 42, adynamic-characteristic time constant τa estimation section 43, avolumetric-efficiency estimation section 44, an advance-processingarithmetic operation section 45, and a target EGR-valve-openingarithmetic operation section 46. The target EGR amount per cylindersetting section 41 acts to set the first target EGR amount MQce. Theresponse-characteristic time constant τs setting section 42 functions toset a response characteristic (a time constant τs) between the firsttarget EGR amount MQce and the actual EGR amount Qce. Thedynamic-characteristic time constant τa estimation section 43 functionsto estimate a dynamic characteristic (a time constant τa) from the timewhen the recirculated exhaust gas flow passes through the EGR controlvalve to the time when the recirculated exhaust gas flow is drawn intothe inlet of the engine cylinder, on the basis of both the engine speedNe detected by the sensor 17 and a volumetric efficiency whichefficiency is estimated by the volumetric-efficiency estimation section44. The advance-processing arithmetic operation section 45 functions tocalculate the second target EGR amount (the advance-processed target EGRamount) M₂ Qe from the first target EGR amount MQce through the advanceprocessing based on the dynamic characteristic (the time constant τa)estimated by the dynamic-characteristic time constant τa estimationsection 43 so that the response characteristic (the time constant τs)set by the setting section 42 is reached. The target EGR-valve-openingarithmetic operation section 46 derives a target EGR valve opening or atarget fluid-flow passage area (correlated to a target lift of the EGRvalve) from the second target EGR amount M₂ Qe. Thevolumetric-efficiency estimation section 44 estimates a volumetricefficiency η_(v) from both the engine speed Ne and the collectorinternal-pressure Pcol. Actually, the volumetric efficiency η_(v) isretrieved from the engine speed Ne and the collector internal-pressurePcol in accordance with the experimentally determined data map as shownin FIG. 27. For instance in case of a four-cylinder diesel engine, thesystem of the modification shown in FIG. 26 operates as follows.

In the system of the modification, the dynamicc-haracteristic timeconstant τa estimation section 43 estimates a dynamic characteristic (atime constant τa) from the engine speed Ne and the estimated volumetricefficiency η_(v), whereas the response-characteristic time constant τssetting section 42 sets a response characteristic (a time constant τs)such that the response-characteristic time constant τs is a positivenumber less than the dynamic-characteristic time constant τa (see thefollowing inequality).

    0<τs<τa                                            (3)

The advance-processing arithmetic operation section 45 of the systemshown in FIG. 26 calculates the advance-processed target EGR amount (thesecond target EGR amount M₂ Qe) in accordance with the followingLaplace-transformation operation expression (4), using theresponse-characteristic time constant τs, the dynamic-characteristictime constant τa, and the first target EGR amount MQce.

    M.sub.2 Qe={(1+τa·s)/(1+τs·s)}·MQce(4)

where s denotes a Laplace operator.

As a result of the above Laplace-transformation operation expression(4), an actual EGR amount Qce which will be actually drawn into thecylinder, is expressed as the following approximate expression (5) onthe assumption that the above-noted second target EGR amount M₂ Qe isequal to the EGR amount Qce actually passing through the EGR valve.

    Qce={1/(1+τs·s)}·MQce                (5)

FIG. 28 shows simulation results of the actual EGR amount Qce when aso-called step input is applied as the first target EGR amount MQce, inthe presence of the advance-processing of the expression (4) with thetime constant τs set at 0.05 sec and the time constant τs set at 0.13sec and in the absence of the processing of the expression (4). As canbe appreciated from the simulation results of FIG. 28, in the presenceof the processing of the expression (4), the responsibility of the EGRcontrol is enhanced. As appreciated from the approximate expression (5),the smaller the response-characteristic time constant τs, the greaterthe responsibility of the EGR control. However, if the time constant τsis set at an excessively less value, the amplitude of the second targetEGR amount M₂ Qe (=the actual EGR amount Qce) becomes extremely greateras compared with the first target EGR amount and thus there is anincreased tendency for the required opening of the EGR valve to becomeexcessively greater. In this case, the actual EGR amount Qce mayovershoot the first target EGR amount MQce. Therefore, it is preferablethat the response-characteristic time constant τs is set at a minimumpossible value in view of a maximum possible opening of the EGR valve 9.As set forth above, since the system of the modification shown in FIG.26 can more precisely perform the advance processing in consideration ofthe dynamic characteristic of the recirculated exhaust gas so that thedesired response characteristic is satisfied, and thus a high-precisionand high-stability EGR control can be assured. Furthermore, since thedynamic-characteristic time constant ra and the response-characteristictime constant τs (0<τs<τa) are determined or estimated as explainedpreviously, there is less hunting (overshoot and/or undershoot withrespect to the target EGR amount) of the EGR control. Moreover, therequired fluid-flow area Tav is accurately calculated as a function ofthe required EGR amount (or the command EGR amount) and the differentialpressure (Pexh-Pm) between the exhaust pressure and the intake pressure,and additionally another advance processing reflecting the delay inactuating timing of the EGR valve is made to the target valve lift Mliftequivalent to the required fluid-flow area Tav so as to produce thecommand valve lift Liftt (the control signal value required for thetarget EGR valve opening), thus ensuring a high-precision openingcontrol of the EGR valve.

Second Embodiment

Referring now to FIGS. 29 to 35, the automotive emission control systemof the second embodiment is exemplified in case of a diesel engine. Thebasic construction of the system of the second embodiment shown in FIGS.29 to 35 is similar to that of the first embodiment shown in FIGS. 1 to25. Therefore, the same reference numerals used in the first embodimentof FIG. 1 will be applied to the corresponding elements used in thesecond embodiment of FIG. 29, for the purpose of comparison between thefirst and second embodiments. The second embodiment is different fromthe first embodiment in that the opening of an intake-air throttle valve70 (see FIG. 30) is accurately controlled variably depending on at leastthe differential pressure between the exhaust pressure (Pexh) and theintake pressure (Pm), and the actual lift (Lifts) of the EGR valve 9, inaddition to the previously-noted EGR valve control. As seen in FIG. 29,the engine system 5 is equipped with the EGR passage 10 which recycles asmall part of the inert exhaust gas back to the intake manifold 4. TheEGR valve 9 is disposed in the EGR passage 10 for controlling the amountof the recirculated exhaust gas from the exhaust manifold 8 to theintake manifold. The EGR valve 9 is comprised of a valve 50, a valvestem 51 whose one end is fixedly connected to or integrally formed withthe valve 50, a diaphragm 52 fixedly connected to the other end of thevalve stem 51, a return spring 53 which biases the diaphragm downwards(viewing FIG. 29) in a manner so as to keep the valve in its fullyclosed position, a signal line 54, and a diaphragm chamber 55. Asalready explained in the accompanying FIG. 1, the signal line 54 of theEGR valve 9 is connected to the outlet port of the duty-cycle controlledelectromagnetic valve 12, and thus the vacuum which is generated by avacuum source (the vacuum pump 11) and suitably diluted with theatmosphere is fed from the valve 12 via the line 54 into the diaphragmchamber 55. Thus, depending on the degree of the incoming vacuum, theEGR valve can be raised or lowered. The intake-air throttle valve 70(see FIG. 30) is disposed in the induction passage communicating theintake manifold 4 for properly throttling or constricting the inducedfresh air flow. Disposed in the induction passage is the intake-pressuresensor 35. Also disposed in the exhaust passage (the exhaust manifold 8)is an exhaust-pressure sensor 56. In order to perform both the EGRcontrol and the intake-air throttle-valve-opening control, a controlunit 60 is provided. The input interface of the control unit 60 isconnected to the air-flow meter 16, the engine-speed sensor 17 and theaccel-opening sensor 57, to receive the induced fresh-air flowindicative voltage signal Qo from the air-flow meter 16, theengine-speed indicative signal Ne from the sensor 17, and theaccelerator opening indicative signal Acc from the sensor 57. As seen inthe block diagram of FIG. 30, the control unit 60 (indicated by theone-dotted line H in the block diagram) employed in the system of thesecond embodiment includes a desired EGR flow rate arithmetic operationsection B, an EGR valve lift setting section C, an EGR valve controlsection D, an EGR valve lift detection section E, an intake-airthrottle-valve-opening setting section F and an intake-air throttlevalve control section G. The operation section B is connected to anengine operating-state detection section A, for calculating a desiredEGR flow rate (a target EGR flow rate) on the basis of a plurality ofengine operating-state indicative signals from the detection section A,namely the engine speed indicative signal Ne, the accel-openingindicative signal Acc, the intake pressure Pm, the exhaust pressure Pexhand the like. The setting section C determines a desired lift (a setpoint) of the EGR valve 9 on the basis of the desired EGR flow ratecalculated by the section B. The control section D controls the EGRvalve 9 on the basis of the set point determined by the section C. Thedetection section E is provided to detect an actual lift of the valve 9.The throttle-valve-opening setting section F is provided for setting adesired opening of the throttle valve 70 depending on all the engineoperating-state indicative signal, the desired EGR flow rate, and theactual lift of the EGR valve. The control section G controls thethrottle valve 70 in response to the desired throttle-valve-openingindicative signal from the setting section F. As detailed herebelow, thesystem of the second embodiment executes the throttle-valve-openingcontrol as well as the same EGR control as the first embodiment.

Referring now to FIG. 31, there is shown a control flow for the openingof the intake-air throttle valve 70. In step S201, a maximum EGR flowrate Qemax (as will be explained later with respect to the flowindicated in FIG. 33), the actual lift Lifts of the EGR valve 9, and thedifferential pressure Dpl (=Pexh-Pm) between the exhaust pressure Pexhand the intake pressure Pm are read. In step S202, the required EGR flowrate Tqe is compared with the difference (Qemax-QOFF#) which is obtainedby subtracting a predetermined value QOFF# from the maximum EGR flowrate Qemax. In case that the inequality Tqe>Qemax-QOFF# is satisfied,step S205 proceeds in which the current value Th (Thn) of athrottle-valve set parameter (a set point) is updated by a value(Th_(n-1) -1) obtained by subtracting "1" from the previous valueTh_(n-1) of the set parameter so that the opening TVO of the throttlevalve 70 decreases, since the desired EGR flow rate exceeds the maximumEGR flow rate Qemax. The predetermined value QOFF# is preset inconsideration of fluctuations of the EGR flow rate, resulting from theEGR valve characteristics. If the answer to step S202 is negative (NO),i.e., in case of Tqe≦Qemax-QOFF#, step S203 enters in which a test ismade to determine whether the actual valve lift Lifts is less than apredetermined constant LIFTSL#. In case of Lifts<LIFTSL#, step S204proceeds in which the differential pressure Dpl (=Pexh-Pm) is comparedwith a predetermined constant or a predetermined slice level DPLSL#. Incase of Lifts≧LIFTSL#, step S207 proceeds. When the answer to step S204is affirmative (YES), step S206 enters. Conversely when the answer tostep S204 is negative, step S207 proceeds. The flow from step S203 viastep S204 to step S206 means that the actual valve lift Lifts iscomparatively less and additionally the differential pressure Dpldevelops sufficiently, and thus in step S206 the current value Th (Thn)of the throttle-valve set parameter is updated by a value (Th_(n-1) +1)obtained by adding "1" to the previous value Th_(n-1) of the setparameter so that the opening TVO of the throttle valve 70 increases.The flow from step S202 via S203 or step S204 to step S207 means thatthe desired EGR flow rate is within a permissible range, and the actuallift of the EGR valve is almost satisfied or there is a proper level ofthe differential pressure Dpl, and thus the control unit 60 decides thatthe current opening of the throttle valve is proper. For the reason setfor above, in step S207 the current value Th (Thn) of the throttle-valveset parameter is held at the same value as the previous value Th_(n-1)so that the opening TVO of the throttle valve 70 retains unchanged. Instep S208, the upper and lower limits of the set parameter Th of thethrottle valve opening are restricted respectively by "1" and apredetermined maximum possible valve opening set number STVO#, asexpressed by the inequality 1≦Th≦STVO#. In step S209, the throttle-valveopening TVO is controlled on the basis of the set parameter finallydetermined through step S208. In the second embodiment, the relationbetween the throttle valve opening TVO and the set parameter Th isdetermined by the characteristic curve shown in FIG. 32. Thecharacteristic curve is predetermined in a manner to be able to moreaccurately minutely set the opening TVO in the part-throttle mode,because the EGR is required mainly during a comparatively low enginespeed with the throttle valve partly opened, and in such a case a slightchange in the throttle-valve opening may produce a great rate of changein the induced air flow.

Referring to FIG. 33, there is shown a routine for calculation of themaximum EGR flow rate Qemax. Through steps S211 and S212, the intakepressure Pm and the exhaust pressure Pexh are read. In step S213, thedifferential pressure Dpl is calculated as the difference (Pexh-Pm)between the exhaust pressure and the intake pressure. In step S214, aflow velocity Vqe of the recirculated exhaust gas flow is derivedaccording to the following expression.

    Vqe=(Dpl).sup.1/2 ×KR#×Te/TA#

where KR# is the predetermined constant, Te denotes the EGR temperature,and TA# denotes the predetermined standard temperature.

In step S215, in case of the maximum possible lift of the EGR valve, themaximum opening area Avmax of the EGR passage or the EGR valve isretrieved from the look-up table as shown in FIG. 14. The, the maximumEGR flow rate Qemax is calculated as the product (Avmax×Vqe) of themaximum opening area Avmax and the recirculated exhaust-gas flowvelocity Vqe.

Referring to FIG. 34, there is shown another control flow for theopening of the intake-air throttle valve 70. The control flow shown inFIG. 34 is slightly different from the control flow shown in FIG. 31only in that steps S203 and S204 of FIG. 31 is replaced with step S223of FIG. 34. A comparative value Liftsl used in step S223 is a slicelevel which is variable depending on the previously-noted flow velocityVqe, as may be appreciated from the flow chart of FIG. 35. In otherwords, the value Liftsl is a flow-velocity dependent variable. Actually,since the flow velocity Vqe itself is expressed as a function(Vqe=(Dpl)^(1/2) ×KR#×Te/TA#) of the differential pressure Dpl, it willbe appreciated that the slice level Liftsl based on the flow velocityreflects the decision box S204 as well as the decision box S203. In stepS231 of FIG. 35, the flow velocity Vqe is read. In step S232, the slicelevel Liftsl is retrieved from a predetermined look-up table (not shown)indicative of the relationship between the flow velocity Vqe and theslide level Liftsl of the valve lift. The respective routines shown inFIGS. 31, 33 and 34 are executed as time-triggered interrupt routines tobe triggered every predetermined time intervals for example 10 msec.

As clearly seen in FIGS. 36A to 36E, according to the throttle-valveopening control of the system of the second embodiment, in case that thedesired EGR flow rate (the required EGR flow rate) Tqe exceeds themaximum EGR flow rate Qemax, the throttle-valve opening is decreasinglyadjusted. Also, in the event that the differential pressure Dpl is abovethe predetermined constant DPLSL# when the desired EGR flow rate Tqe isless than the maximum EGR flow rate Qemax, the throttle-valve opening isincreasingly adjusted. As compared with the prior system (indicated bythe broken line in FIG. 36D and 36E), the system of the invention cankeep the excess air factor at an approximately constant level.

According to the system of the second embodiment, since the opening ofthe intake-air throttle valve is controlled depending on thedifferential pressure between the exhaust pressure and the intakepressure and the actual lift of the EGR valve, it is possible toaccurately and minutely control the throttle-valve opening, quicklyresponding to the environmental change or the change in operatingconditions of the vehicle, and thus all of engine performance, fuelconsumption, and exhauste-mission control performance are properlyharmonized with one another. Furthermore, in the system of the secondembodiment, the throttle-valve opening can be automatically optimized bydetermining only the flow-rate characteristic of the EGR valve based onthe previously-noted differential pressure and the actual lift of theEGR valve. Therefore, the capacity of a built-in read only memory (ROM)mounted on the control unit and workhours requiring for production ofthe system can be greatly reduced. Moreover, the maximum EGR flow rate(Qemax) is derived from the above-mentioned differential pressure (Dpl)and the maximum opening area (Avmax) of the EGR passage (or the maximumfluid-flow passage area of the EGR valve) determined by the maximumpossible lift of the EGR valve, and the throttle-valve opening isdecreasingly adjusted or compensated usefully and timely in the eventthat the desired EGR flow rate (Tqe) exceeds the maximum EGR flow rate(Qemax). Additionally, the throttle-valve opening is increasinglyadjusted or compensated in the event that the above-noted differentialpressure (Dpl) is above a predetermined constant (DPLSL#) and/or theactual lift (Lifts) of the EGR valve is below a predetermined constant(LIFTSL#). This controlling operation for the throttle valve is much tothe point, and thus the throttle-valve opening control may be optimizedeven in the presence of the environmental change or the change in theengine operating conditions.

Third Embodiment

Referring now to FIGS. 37 to 42 and FIGS. 43A to 43F, the automotiveemission control system of the third embodiment is exemplified in caseof a diesel engine with a turbocharger. The basic construction of theEGR control system employed in the emission control system of the thirdembodiment is similar to that of the first embodiment shown in FIGS. 1to 25. Only the EGR control routine which is executed by a control unitincorporated in the system of the third embodiment, is different fromthe first embodiment. For the purpose of simplification of thedisclosure, only the EGR-control routine of the third embodiment will behereinbelow discussed in detail.

The control unit employed in the system of the third embodiment performsan EGR control in accordance with the flow shown in FIG. 37.

In step S241, an engine operating-state indicative data is read.Concretely, an engine-load representative data such as an accel-openingindicative signal Acc and a fuel-injection amount Qsol, and anengine-speed indicative data Ne are read as the engine operating-stateindicative data. In step S242, the intake pressure Pm is read. Theintake pressure Pm is measured or detected directly by means of theintake-pressure sensor 35 located in the intake manifold or in theinduction passage. Alternatively, the intake pressure Pm can be derivedthrough the routine for the arithmetic operation as shown in FIG. 2. Instep S243, a target EGR lift (a desired EGR lift) of the EGR valve 9 iscalculated on the basis of both the engine operating-state indicativedata and the intake pressure Pm. In step S244, the EGR valve is drivenor the opening of the EGR valve is controlled, such that the targetvalve lift derived in step S243 is reached. In the case that theduty-cycle controlled electromagnetic solenoid valve as shown in FIG. 1is utilized for driving the valve lift mechanism (including thediaphragm chamber) of the EGR valve, the duty factor of the solenoidvalve 12 is set at a proper duty ratio based on the deviation betweenthe actual lift of the EGR valve and the target valve lift.Alternatively, in case that a stepper motor is used for adjusting thelift of the EGR valve, the angular steps of the stepper motor are set atproper steps based on the above-mentioned deviation.

Referring to FIG. 38, there is shown a sub-routine for calculation ofthe desired lift or target lift (Tlift) of the EGR valve. In step S251,en engine-load equivalent value Qfe is calculated as a function of theengine load representative data and the intake pressure Pm. The valueQfe is obtained by dividing a value of the engine-load representativedata by the intake pressure Pm. That is, the value Qfe is expressed asQfe=(a value of the engine-load representative data)/(the intakepressure Pm). As may be appreciated from this expression, in the eventthat the fuel-injection amount (Qsol) rapidly increases in a transientengine operating state such as when heavily accelerating, the value ofthe engine-load representative data tends to be increased relative tothe intake pressure Pm owing to the delay in the change in the intakepressure. In step S252, a target EGR-valve lift Tlift is retrieved fromboth the engine speed Ne and the engine-load equivalent value Qfe, inaccordance with the characteristic curves shown in FIG. 40. The enginespeed Ne versus engine-load equivalent value Qfe characteristic curvesare stored in a memory of the control unit in the form of the data map.

Referring now to FIG. 39, there is shown another sub-routine forcalculation of the target EGR-valve lift (Tlift). In step S261, a targetintake pressure Pmt is derived or retrieved from both the engine-loadrepresentative data such as the accel-opening indicative signal Acc, thefuel-injection amount Qsol or the like, and the engine speed indicativedata Ne, in accordance with the data map shown in FIG. 42, which map isexperimentally determined. In step S262, a differential pressure dPm(=Pm-Pmt) between the detected or calculated intake pressure Pm which isregarded as the actual intake pressure and the target intake pressurePmt. In step S263, an engine-load dependent correction factor Kqf isderived or retrieved from the differential pressure dPm by reference tothe look-up table as shown in FIG. 41. In step S264, the engine-loadequivalent value Qfe is derived from both the engine-load representativedata and the engine-load dependent correction factor Kqf, according tothe expression Qfe=(a value of the engine-load representative data)×Kqf.In step S265, the target EGR-valve lift Tlift is retrieved from both theengine speed Ne and the engine-load equivalent value Qfe, by referenceto the look-up table as shown in FIG. 40.

As can be appreciated from the two routines shown in FIGS. 37 and 39,according to the system of the third embodiment, the target EGR rate (orits equivalent value such as the target EGR-valve lift Tlift) isproperly corrected depending on the intake pressure Pm, and thus it ispossible to perform an optimal EGR control even in a transient engineoperating condition such as when rapidly accelerating. This prevents theexhaust-emission control performance from being deteriorated during thetransient operating mode. The previously-discussed system of the thirdembodiment can provide a high-precision EGR control particularly in caseof a diesel engine with a turbocharger. Hitherto the target EGR rate orits corresponding value (the EGR valve lift) is set on the suppositionthat the intake pressure is a standard pressure such as a predeterminedpressure level PA#, and thus the target EGR rate calculated through theprior art EGR control system corresponds to a desired value under aparticular condition in which the intake pressure reaches the standardpressure, and thus there is a tendency of an insufficient amount ofinduced fresh air until the intake pressure reaches the standardpressure. In such a case it is necessary to decreasingly correct thetarget EGR rate derived through the prior art system. On the other hand,in the system of the third embodiment, since the engine-load equivalentvalue Qfe tends to become greater relative to the intake pressure in atransient engine operating condition such as during acceleration. Asappreciated from the data map shown in FIG. 40, the greater theengine-load equivalent value Qfe, the smaller the target EGR-valve liftTlift. That is to say, the target EGR-valve lift (essentiallycorresponding to the EGR rate) is suitably corrected depending on thechange in the intake pressure Pm. Accordingly, even in a transientengine operating condition such as during hard acceleration, the systemof the third embodiment can insure an optimal high-precision EGRcontrol. Referring to FIGS. 43A to 43F, there is shown timing chartsexplaining effects obtained by the system of the third embodiment. As iswell known, the target EGR rate (or the target EGR amount) is determinedin consideration of the trade-off between suppression of formation ofNOx emissions and the formation of particulates emitted from the exhaustsystem or between the increase in NOx emissions and the decrease inparticulates. A set point of the EGR rate is generally determined thatthere is a comparatively low sensitiveness of formation of NOx emissionsand that there is a comparatively high sensitiveness of formation ofparticulates. For the reasons set out above, as compared with the priorart system, in case of the system of the third embodiment there is atendency that NOx emissions tend to slightly increase whereas there is atendency that particulates tend to remarkably decrease in a transientengine operating condition, i.e., in case of a rapid increase in engineload (the accel-opening Acc or the fuel-injection amount Qsol).

Fourth Embodiment

Referring now to FIGS. 44 to 53, the automotive emission control systemof the fourth embodiment is exemplified in case of a diesel engine witha turbocharger. The basic construction of the system of the fourthembodiment is similar to that of the first embodiment shown in FIGS. 1to 25. Only the fuel-injection amount (Qsol) arithmetic-operationroutine which is executed by a control unit incorporated in the systemof the fourth embodiment, is different from the first embodiment. Thatis, in the system of the fourth embodiment, a sub-routine for moreprecise correction of the fuel-injection amount is additionally insertedinto the arithmetic operation for the fuel-injection amount Qsol. Forthe purpose of comparison between the systems of the first and fourthembodiments, such fuel-injection amount correction sub-routines will behereinbelow explained in detail.

Referring now to FIG. 44, the control unit employed in the system of thefourth embodiment performs a fuel-injection amount control as follows.

In step S271, read are the engine speed Ne and the control-lever openingCL of the injection pump 7. In step S272, a basic fuel-injection amountMqdrv is retrieved from both the engine speed Ne and the control-leveropening CL, according to the look-up table as shown in FIG. 22. In stepS273, the basic fuel-injection amount Mqdrv is corrected by variouscorrection factors such as a water-temperature dependent correctionfactor and the like, to produce a primarily corrected fuel-injectionamount Drvq. In step S274, the primarily corrected fuel-injection amountDrvq is corrected again according to a correction sub-routine as shownin FIG. 45, so as to produce a secondarily corrected fuel-injectionamount Qsolb. In step S275, in the event that the secondarily correctedfuel-injection amount Qsolb exceeds an upper limit (a given maximumfuel-injection amount Qful as calculated through another sub-routineshown in FIG. 50), the corrected fuel-injection amount Qsolb is replacedwith the upper limit to keep the actual output value of thefuel-injection amount Qsol within the upper limit. When the secondarilycorrected fuel-injection amount Qsolb is below the upper limit, thecorrected fuel-injection amount Qsolb is regarded as the actual outputvalue of the injection amount Qsol.

Referring now to FIG. 45, there is shown one sub-routine for correctionof the fuel-injection amount.

In step S281, read is the target EGR rate Megr as already discussed inthe accompanying FIGS. 19 and 20. In step S282, read is an actual EGRrate Regr of exhaust gas recirculated through the EGR valve. The actualEGR rate Regr is usually obtained in the form of the actual EGR-valvelift Lifts directly detected by way of the EGR-valve lift sensor 34 asshown in FIG. 1. In step S283, the deviation dEGR (=Megr-Regr) betweenthe target EGR rate Megr and the actual EGR rate Regr is calculated. Instep S284, a fuel-injection-amount correction factor Kqsolh is retrievedfrom the EGR-rate difference dEGR in accordance with the look-up tableas shown in FIG. 46. In step S285, read is the primarily correctedfuel-injection amount Drvq is read. In step S286, calculated is thedeviation Dtq (=Drvq-Qsoln-1) between the primarily correctedfuel-injection amount Drvq and the previous value Qsoln-1 of thefuel-injection amount. In step S287, the secondarily correctedfuel-injection amount Qsolb is derived from both the deviation Dtq andthe fuel-injection-amount correction factor Kqsolh, according to thefollowing expression.

    Qsolb=Qsoln-1+Dtq×Kqsolh

As appreciated from the EGR-rate deviation (dEGR) versusfuel-injection-amount correction factor (Kqsolh) characteristic curveshown in FIG. 46, the correction factor Kqsolh is so designed to be setat "1" when the deviation DEGR of the EGR rate is "0" and to begradually decreased down to a predetermined decimal fraction less than"1" and slightly greater than "0" along a substantially parabolic curveas the absolute value |dEGR| of the EGR-rate deviation dEGR increases,and to be held at the above-mentioned predetermined decimal fractionwhen the EGR-rate deviation dEGR exceeds its predetermined upper orlower limit. For instance, in case that the EGR-rate deviation dEGR is"0", the correction factor Kqsolh is "1". In this case, the secondarilycorrected fuel-injection amount Qsolb becomes equal to the primarilycorrected fuel-injection amount Drvq, becauseQsolb=Qsoln-1+Dtq×Kqsolh=Qsoln-1+Dtq=Qsoln-1+Drvq-Qsoln-1=Drvq.

Referring now to FIG. 47, there is shown another subroutine forcorrection of the fuel-injection amount.

In step S291, the volumetric-efficiency equivalent value Kin is read.The volumetric-efficiency equivalent value Kin can be calculated as theproduct (Kinn×Kinp) of the engine-speed dependent retrieved correctionfactor Kinn (see FIG. 10) and the intake-pressure dependent retrievedcorrection factor Kinp (see FIG. 9), essentially in the same manner asexplained previously in FIGS. 8, 9 and 10. In step S292, the secondarilycorrected fuel-injection amount (a final fuel-injectionamount/cylinder/intake stroke) Qsolb is estimated according to thefollowing first-order-lag expression.

    Qsolb=Qsoln-1×(1-Kvol×Kin)+Drvq×Kvol×Kin

where Kvol denotes the previously-noted predetermined volumetric ratio(Vc/Vm) of the volumetric capacity/cylinder (Vc) with respect to thecollector and intake-manifold volumetric capacity Vm in the inductionsystem, the product (Kvol×Kin) means what percent of the primarilycorrected fuel-injection amount Drvq currently calculated is actuallydrawn into the cylinder. Therefore, owing to the first-order lag, thefirst term {Qsol_(n-1) ×(1-Kvol×Kin)} corresponds essentially to therate of the fuel-injection amount which will be drawn into the cylinderfrom within the previous value Qsol_(n-1) of the fuel-injection amountcalculated at the previous arithmetic-operation cycle (see FIG. 44),while the second term (Drvq×Kvol×Kin) corresponds essentially to therate of the fuel-injection amount which will be drawn into the cylinderfrom within the primarily corrected fuel-injection amount Drvq derivedat the current arithmeticoperation cycle (see step S273 of FIG. 44).

Referring now to FIGS. 48 and 49, there is shown another sub-routine forcorrection of the fuel-injection amount.

In step S301, the induced fresh-air flow rate Qas0 is read. The flowrate Qas0 is obtained as the weighted mean of the intake-air flow rateQasm through the flow from step S21 via step S22 to step S23 aspreviously explained in FIG. 4. In the shown embodiment, the intake-airflow rate Qasm is derived from the voltage signal value Qo from theair-flow meter, in accordance with the look-up table as shown in FIG.53. In step S302, the induced fresh-air flow/cylinder/induction-stroke(abbreviated simply the fresh-air flow per cylinder Qac) is calculatedaccording to the following expression.

    Qac=Qas0/Ne×120/CYLN#

where Qas0 denotes the weighted mean of the intake-air flow Qasm, Nedenotes the engine speed, and CYLN# denotes the number of enginecylinders.

In step S303, an allowable fuel-injection amount Qsolc is derived fromthree parameters, namely the excess-air-factor equivalent value Lamb(calculated at step S307 one cycle before), the allowableexcess-air-factor variation Dlamb (retrieved at step S308 one cyclebefore), and the fresh-air flow per cylinder Qac, in accordance with thefollowing expression.

    Qsolc=Qac/(Lamb-Dlamb)/14.6

In step S304, the allowable fuel-injection amount Qsolc is compared withthe primarily corrected fuel-injection amount Drvq. In case ofQsolc≧Drvq, step S305 proceeds in which the primarily correctedfuel-injection amount Drvq is regarded as the finally correctedfuel-injection amount Qsolb. In contrast, in case of Qsolc<Drvq, stepS306 proceeds in which the allowable fuel-injection amount Qsolc isregarded as the finally corrected fuel-injection amount Qsolb. In otherwords, smaller one of the two calculated fuel-injection amounts Drvq andQsolc is selected. In step S307, the excess-air-factor equivalent valueLamb is calculated as a function of both the finally correctedfuel-injection amount Qsol (precisely Qsolb) and the fresh-air flow percylinder Qac, in accordance with the following expression.

    Lamb=Qac/Qsol/14.6

In step S308, the allowable excess-air-factor variation Dlamb is derivedor retrieved from the excess-air-factor equivalent value Lamb calculatedat step S307, by reference to the look-up table as shown in FIG. 49. Asappreciated from the characteristic curve shown in FIG. 49, theallowable excess-air-factor variation Dlamb is preset to besubstantially proportional to the magnitude of the excess-air-factorequivalent value Lamb, with the result that the exhaust-emission controlperformance and the driveability are properly balanced each other.

Referring now to FIG. 50, there is shown the sub-routine for calculationof the maximum fuel-injection amount Qful.

In step S311, the fresh-air flow per cylinder Qac is read. Precisely, inaddition to the fresh-air flow per cylinder Qac, the engine speed Ne andthe intake pressure (precisely the intake pressure Pm_(n-1) derived onecycle before) are read in step S311.

In step S312, the limit excess-air factor Klamb is firstly determined asthe product (Klambn×Klambp) of the engine-speed dependent retrievedfactor Klambn (see the look-up table shown in FIG. 51) and theintake-pressure dependent retrieved factor Klambp (see the look-up tableshown in FIG. 52). Then, the maximum fuel-injection amount Qful iscalculated as a function of the fresh-air flow per cylinder Qac and thelimit excess-air factor Klamb (=Klambn×Klambp), in accordance with thefollowing expression.

    Qful=Qac/Klamb/14.6

According to the system of the fourth embodiment, the fuel-injectionamount is accurately corrected depending on the engine operatingconditions such as the presence or absence of the exhaust-gasrecirculation, and the change in the EGR rate (Megr), thus preventingthe excess-air factor from greatly lowering unintendedly. As aconsequence, the derivability (an accelerating performance) and theemission control performance are well-balanced each other. Furthermore,the fuel-injection amount is precisely corrected depending on a desiredfuel-injection amount (Drvq), the target EGR rate and the actual EGRrate, (particularly the deviation (dEGR) between the target EGR rate andthe actual EGR rate), and thus the delivery of fuel-injection amount isoptimized. Moreover, in the first modification of the injection-amountcorrection, in consideration of a first-order lag until thefuel-injection amount instantaneously calculated is actually deliveredinto the cylinder, the desired injection amount (corresponding to theprimarily corrected fuel-injection amount) is further corrected. Thisenhances an accuracy of the injection-amount correction. In order tomore precisely correct the injection amount in the previously-notedmodification, uses as a first-order-lag time constant (Kvol×Kin) aplurality of parameters, namely the collector and intake-manifoldvolumetric capacity Vm, the volumetric capacity/cylinder Vc, and thevolumetric-efficiency equivalent value Kin based on the engine speed Neand the intake pressure Pm_(n-1). The first-orderlag time constant maybe determined by at least one of these parameters Vm, Vc, Kin and thelike. Additionally, in the second modification of the injection-amountcorrection, the fuel-injection amount can be precisely correcteddepending on the desired fuel-injection amount (Drvq) and the calculatedexcess-air factor (Lamb). Particularly, as clearly seen in FIG. 49, thefuel-injection amount can be more precisely corrected and adjusted inconsideration of the allowable excess-air-factor variation (Dlamb)estimated from and thoroughly correlated to the calculated excess-airfactor (Lamb), and as a result the actual fuel-injection amount isproperly adjusted depending on the magnitude of the calculatedexcess-air factor Lamb so that the variation in the excess-air factordoes not extremely increase, thereby preventing the excess-air factorfrom rapidly lowering. Thus, the exhaust-emission control performanceand the driveability are properly well-balanced each other.

Fifth Embodiment

Referring now to FIGS. 54 to 72, there is shown a main routine forarithmetic operation of an averaged intake-air flow rate or a weightedmean of the induced fresh-air flow rate.

In step S341, the voltage signal Qo from the air-flow meter 16 (seeFIG. 1) is read. In step S342, the voltage signal Qo is converted intoan intake-air flow rate (or an induced fresh-air flow rate) Qas01 by wayof linearization executed according to the conversion table as shown inFIG. 53. In step S343, in consideration of the response delay orresponse lag inherent in the air-flow detecting device such as ahot-wire type air flow meter, a so-called advance processing (or a leadprocessing) is made to the induced fresh-air flow rate Qas01 to producean advance-processed fresh-air flow rate Qas02. In step 344, areverse-flow detection is performed utilizing the advance-processedfresh-air flow rate Qas02, and simultaneously a flowrate correction isperformed on the basis of a result of the reverse-flow detection, toproduce a corrected fresh-air flow rate (or a reverse-flow correctedvalue) Qas03. In step S345, an averaging process is made with respect tothe corrected fresh-air flow rate Qas03 to produce the averagedintake-air flow rate Qas0. The advance processing of step S343 isherein-below described in detail with respect to the sub-routine shownin FIG. 55. The advance-processing indicated in FIG. 55 is executed astime-triggered interrupt routines to be triggered every predeterminedtime intervals such as 4 msec. As may be appreciated from theair-flow-meter response characteristic curve shown in FIG. 56 (astep-response test data) which is experimentally assured by theinventor, the typical hot-wire type air-flow meter has a first timeconstant (or a first lag coefficient) T1 as indicated by the zone A anda second time constant (a second lag coefficient) T2 as indicated by thezone B.

Referring now to FIG. 55, in step 351, in consideration of the firsttime constant T1, a first advance processing is performed in accordancewith the following expression.

    Qa11=Qas01.sub.n-1 +(Qas01-Qas01.sub.n-1)×0.004/T1

where Qas01n-1 denotes the previous value of the induced fresh-air flowrate converted at step S342, Qas01 denotes the current value of theinduced fresh-air flow rate, and T1 denotes the first time constant.

In step S352, in consideration of the second time constant T2, a secondadvance processing is performed to produce asecondarily-advanced-processed fresh-air flow rate Qas02, in accordancewith the following expression.

    Qas02=Qa11n-1+(Qa11-Qa11n-1)×0.004/T2

where Qa11n-1 denotes the previous value of theprimarily-advance-processed fresh-air flow rate calculated at step S351one cycle before, Qas11 denotes the current value of theprimarily-advance-processed fresh-air flow rate calculated in step S351at the current arithmetic-operation cycle, and T2 denotes the secondtime constant.

Referring now to FIGS. 57 and 58, there is shown a flow chart requiredfor the reverse-flow detection and the flow-rate correction.

In step S361, calculated is a comparative value (or an upper slicelevel) Qa2sl which is compared with a maximal value of the inducedfresh-air flow rate, considering the engine operating conditions. Thecomparative value Qa2sl will be hereinafter described in detail withrespect to the sub-routine shown in FIG. 59.

In step S362, calculated is the variation from the previous valueQas02_(n-1) of the secondarily-advance-processed fresh-air flow ratecalculated through step S352 one cycle before to the currentsecondarily-advance-processed fresh-air flow rate Qas02, that is, thedeviation ΔQa2 (=Qas02-Qas02_(n-1)) between the current flow rate Qas02and the previous flow rate Qas02n-1.

In step S363, a test is made to determine whether the deviation ΔQa2 isa negative number. When the answer to step S363 is affirmative (YES),i.e., in case of ΔQa2<0, step S364 proceeds in which a test is made todetermine whether the previous value ΔQa2_(n-1) of the deviation isgreater than or equal to "0". In case of ΔQa2≧0 in step S363 or in caseof ΔQa2_(n-1) <0 in step S364, the procedure jumps to step S366. Whenthe answer to step S364 is YES (ΔQa2_(n-1) >0), step S365 enters inwhich the pervious deviation ΔQa2_(n-1) is regarded as a maximal valueQa2m, and thus the maximal value Qa2m is updated by the deviationΔQa2_(n-1). In step S363 ΔQa2<0 means that the current flow rate Qas02decreases from the previous flow rate Qas02_(n-1) at the timing of thecurrent arithmetic operation. Also, in step S364 ΔQa2_(n-) 1≧0 meansthat the previous flow rate Qas02n-1 increases from thesecondarily-advance-processed fresh-air flow rate Qas02_(n-2) calculatedat step S352 two cycles before. That is, the flow from step S363 viastep S364 to step S365 means that the previous flow rate Qas02_(n-1)corresponds to a maximal value since the secondarily-advance-processedfresh-air flow rate Qas02 varies from the increasing direction to thedecreasing direction. In the case that the two conditions defined insteps S363 and S364 are unsatisfied simultaneously, the maximal valueQa2m stored in the predetermined memory address in the control unit ofthe system of the fifth embodiment is not updated and as a result theprevious value of the maximal value Qa2m is held as the current value.

In step S366, a test is made to determine whether the previous valueFlags_(n-1) of a sign-decision flag Flags is "1" or "0". When the answerto step S366 is affirmative, i.e., in case of Flags_(n-1) =1, step S370proceeds in which a sign flag Sign is set at "1". Conversely, when theanswer to step S366 is negative (NO), i.e., in case of Flags_(n-1) =0,step S367 enters in which a test is made to determine whether theprevious deviation ΔQa2_(n-1) is a negative number. When the answer tostep S367 is affirmative, i.e., in case of ΔQa2_(n-1) <0, step S368proceeds in which a test is made to determine whether the currentdeviation ΔQa2 is equal to or greater than "0". When the answer to stepS368 is affirmative (YES), the procedure flows to step S370 to set theflag Sign at "1". When the answer to step S368 is negative (NO), stepS369 proceeds in which a test is made to determine whether the maximalvalue Qa2m is equal to or greater than the slice level Qa2sl. When theanswer to step S367 is negative (NO), the procedure flows to step S369to compare the maximal value Qa2m with slice level Qa2sl. When thecondition of Qa2m≧Qa2sl at step S369 is satisfied, step S370 proceeds inwhich the sign flag Sign is set at "1". In case of Qa2m<Qa2sl, step S371proceeds in which the sign flag Sign is set at "-1". Thereafter, in stepS372, the corrected fresh-air flow rate Qas03 is obtained by multiplyingthe previous secondarily-advance-processed fresh-air flow rateQas02_(n-1) by the value of the current sign flag Sign, in accordancewith the following expression.

    Qas03=Qas02.sub.n-1 ×Sign

In step S373, the previous secondarily-advance-processed fresh-air flowrate Qas02_(n-1) is updated by the current secondarily-advance-processedfresh-air flow rate Qas02 so that the current value Qas02 is stored inthe predetermined memory address in the memory of the control unit.

In step S374, a test is made to determine whether the maximal value Qa2mis equal to or greater than the slice level Qa2sl. In case ofQa2m≧Qa2sl, the procedure flows from step S374 to step S376 in which thesign-decision flag Flags is reset at "0". In case of Qa2m<Qa2sl, theprocedure flows from step S374 to step S375 in which a test is made todetermine whether the sign flag Sign is "-1". In case of Sign<0 at stepS375, step S377 proceeds in which the sign-decision flag Flags is set at"1". In case of Sign≧0 at step S375, the value of the sign-decision flagFlags remains unchanged and then one cycle of this sub-routineterminates.

Referring now to FIG. 59, there is shown one arithmetic-operationsub-routine for calculation of the previously-noted comparative value(the slice level) Qa2sl. In step S381, read as the engine operatingcondition is the engine speed Ne. In step S382, the comparative valueQa2sl is derived or retrieved from the engine speed Ne by reference tothe look-up table as shown in FIG. 60. As can be appreciated from thecharacteristic curve shown in FIG. 60, the slice level Qa2sl graduallydecreases as the engine speed Ne increases, because the reverse-flowcomponent included in the voltage signal generated from the air-flowmeter tends to reduce in accordance with the increase in the enginespeed Ne. Also in the conventional system, it is desired to improve theaccuracy of measurement of the induced fresh air flow particularlywithin an engine low-speed range.

Referring now to FIG. 61, there is shown another arithmetic-operationsub-routine for calculation of the previously-noted comparative value(the slice level) Qa2sl. In the same manner as the flow from step S381to step S382 in FIG. 59, in another routine, the engine speed Ne isfirstly read in step S391 and then in step S392 a basic slice level (ora basic comparative value) Qa2slb is retrieved from the engine speed Neby reference to the look-up table as shown in FIG. 60. Thereafter, instep S393, the intake-air throttle-valve opening TVO is read. In stepS394, a throttle-valve-opening dependent slice-level correction factorKqa2sl is retrieved from the intake-air throttle-valve opening TVO, byreference to the look-up table as shown in FIG. 62. In step S395, afinal comparative value or a final slice level Qa2sl is calculated bymultiplying the basic slice level Qa2slb by the correction factorKqa2sl. As may be appreciated from the characteristic curve shown inFIG. 62, the throttle-valve-opening dependent slice-level correctionfactor Kqa2sl is designed so that the slice level Qa2sl is set at acomparatively lower level in the case that the throttle-valve openingTVO is less and thus the induced fresh-air flow is reduced and thereverse-flow component included in the induced fresh-air flow reducesowing to the increase in the flow velocity of the induced fresh-airflow, and so that the slice level Qa2sl is set at a comparatively higherlevel in case of a greater throttle-valve opening wherein there is agreatly increased tendency for the above-mentioned reverse-flowcomponent to increase.

Referring now to FIG. 63, there is shown the arithmetic-operationsub-routine for averaging the reverse-flow corrected fresh-air flow rateQas03. In step S401, in order to produce an averaged intake-air flowrate Qas0, averaged are the current reverse-flow corrected fresh-airflow rate datum Qas03 and the other reverse-flow corrected fresh-airflow rate data Qas03₁, Qas03₂, . . . , Qas03_(N-2), Qas03_(N-1) whichdata stored in predetermined memory addresses from (n-1) cycles before.As clearly indicated in the box of step S402, the data stored in thememory addresses are shifted every cycles. As set forth above, thedetected fresh-air flow rate indicative voltage signal from the air-flowmeter is properly advance-processed in consideration of the response lag(the two time constants T1 and T2) of the air-flow meter, and thereverse-flow of the induced fresh air flow is accurately detected, andthe reverse-flow component included in the output signal from theair-flow meter is precisely corrected, and thereafter the preciselycorrected fresh-air flow rates (Qas03) are averaged in consideration ofpulsation flow of the induced fresh air. As a result of this, theinduced fresh-air flow rate can be precisely calculated or estimated onthe basis of the output from the air-flow meter, thus ensuring ahigh-precision air-fuel-ratio control, irrespective of changes in theengine operating conditions including the environmental change as wellas engine load and engine speed. As previously explained, particularlyin diesel engines, since the EGR control and the fuel-injection-amountcontrol are both based on the induced fresh-air flow rate detected bythe air-floe meter, the reverse-flow corrected and properly averagedfresh-air flow rate (Qas0) can assure a more precise EGR control, thuseffectively reducing NOx emissions and particulates. Furthermore, thefresh-air flow rate (Qas0) precisely calculated can assure a moreprecise fuel-injection-amount control, thus preventing black smoke toincrease.

Referring now to FIG. 64, there is shown a simplified explanatory viewillustrating the reverse-flow correction. The upper half of FIG. 64shows the waveform of the signal advance-processed through step S343,whereas the lower half of FIG. 64 shows the waveform for the signalafter the reverse-flow correction. As appreciated from the flow fromstep S369 via step S371 to step S372 in FIG. 58, when the maximal valueQa2m is less than the slice level Qa2sl, the advance-processed signalQas02 is properly inversed (see the lower half of FIG. 64), and thus theinduced air-flow rate is precisely estimated. As indicated by the brokenline of FIG. 64, the characteristics shown in FIGS. 60 and 62 arepredetermined so that the slice level Qa2sl is set at a value less thana maximal value of a comparatively greater ridge indicating theforward-flow component of the induced fresh air and greater than amaximal value of a comparatively less ridge indicating the reverse-flowcomponent of the induced fresh air. FIG. 65 shows test results in theform of the timing chart in case that the EGR control system is inoperative state in the diesel engine. From the uppermost chart to thelowermost chart, the respective charts respectively indicate the actualintake-air flow rate, the reverse-flow indicative signal, the measuredintake-air flow rate detected by the air-flow meter, the intake-airflow-rate indicative signal generated from the prior art system, theintake-air flow rate obtained through the advance-processing of thepresent invention, and the final output of intake-air flow rateindicative signal obtained through the reverse-flow correctionprocessing. As may be appreciated from comparison of the uppermost chart(the actual intake-air flow rate) and the lowermost chart (the finallycorrected signal after the reverse-flow correction), the finallycorrected signal obtained through the system of the present invention ismore approximate to the actual intake-air flow rate, as compared withthe prior art system. FIGS. 66 shows test results under a particularcondition in which the engine revolution speed is held at 850 rpm, andthe EGR control system is in operative, and the reverse-flow componentincluded in the induced air flow gradually increases from 10 sec afterthe beginning of the test while keeping the amplitude of pulsationsubstantially constant.

As seen in FIG. 66, the prior art system exhibits such a tendency thatits test data (indicated by the one-dotted line) is kept at aconsiderably lower level in the former half of the measurement durationof the induced fresh-air flow, and that the data is kept at considerablyhigher level in the latter half of the measurement duration, as comparedwith the actual flow rate (indicated by the broken line). On the otherhand, the system of the present invention exhibits such a tendency thatthe data (indicated by the solid line) obtained corrections of thepresent invention is essentially approximate to the actual flow rate.

Referring now to FIG. 67, there is shown another routine for decision ofan extreme value (a maximal value or a minimal value) of the outputvoltage signal Qo from the hot-wire type air-flow meter 16 (see FIG. 1).In step S411, the current value Q_(n) of the air-flow meter output Qoand the previous value Q_(n-1) are derived from the predetermined memoryaddresses, and then the deviation D_(n) (=Qn-Qn-1) between the currentvalue Qn and the previous value Q_(n-1) is calculated. In step S412, atest is made to determine whether the current deviation D_(n) is apositive number, i.e., D_(n) >0, and additionally the previous deviationD_(n-1) is a negative number, i.e., D_(n-1) <0. The condition defined bythe inequalities D_(n) >0 and D_(n-1) <0 means that the voltage signalvalue of the air-flow meter varies from a direction of decrease of thevoltage signal to a direction of increase of the voltage signal. In sucha case, the control unit determines that the signal value of the voltagesignal currently generated from the air-flow meter corresponds to aminimal value. When this condition is satisfied, i.e., in case that theanswer to step S412 is affirmative (YES), step S413 proceeds in which aminimal-state indicative flag Flg-min is set at "1". In contrast to theabove, when the answer to step S412 is negative (NO), step S414 proceedsin which the minimal-state indicative flag Flg-min is set at "0", andthen step S415 enters. In step S415, a test is made to determine whetherthe current deviation D_(n) is a negative number (D_(n) <0) andadditionally the previous deviation D_(n-1) is a positive number(D_(n-1) >0). The condition defined by the inequalities D_(n) <0 andD_(n-1) >0 means that the voltage signal value of the air-flow metervaries from a direction of increase of the voltage signal to a directionof decrease of the voltage signal. Thus, the control unit determinesthat the signal value of the voltage signal currently generated from theair-flow meter corresponds to a maximal value. In case that the answerto step S415 is affirmative (YES), step S416 proceeds in which amaximal-state indicative flag Flg-max is set at "1". In contrast to theabove, in case that the answer to step S415 is negative (NO), step S417proceeds in which the maximal-state indicative flag Flg-max is set at"0". Thereafter, the procedure returns to the main routine.

Referring now to FIG. 68, there is shown the sub-routine for countingboth a signal-value increasing time interval C₋₋ Inc and a signal-valuedecreasing time interval C₋₋ Dec with regard to the output voltagesignal Qo from the air-flow meter.

In step S421, a counted value C of the counter (or the timer) is resetto "0". In step S422, the counted value C is incremented by "1". In stepS423, a test is made to determine whether the minimal-state indicativeflag Flg₋₋ min is "1". If the condition of Flg₋₋ min=1 is satisfied,that is, the minimal state is satisfied, step S424 enters in which thesignal-value decreasing time interval C₋₋ Dec is updated by the currentcounted value C. In contrast, when the answer to step S423 is negative(NO), step S425 proceeds in which a test is made to determine whetherthe maximal-state indicative flag Flg₋₋ max is "1". In case that theflag Flg₋₋ max is already set at "1" and thus the control unitdetermines that the maximal state is satisfied at the current timing,step S426 proceeds in which the signal-value increasing time intervalC₋₋ Inc is updated by the current counted value C. In case that thecondition of Flg₋₋ max=1 is unsatisfied, that is, both the answers tosteps S423 and to step S425 are negative, and thus the control unitdetermines that the minimal state and the maximal state are bothunsatisfied, the procedure returns to step S422 to continuously count upthe counted value of the timer. In this manner, the signal-valuedecreasing time interval C₋₋ Dec and the signal-value increasing timeinterval C₋₋ Inc can be precisely measured.

Referring now to FIG. 69, there is shown the simplified explanatorytiming chart related to the sub-routines shown in FIGS. 67 and 68. Theformer-half time period of FIG. 69 shows the simplified waveform of abase signal output from the air-flow meter in absence of the reverseflow in the induction passage, while the latter-half time period of FIG.69 shows the simplified waveform of a base signal output from theair-flow meter in case of occurrence of the reverse flow. In the absenceof the reverse flow (as seen from the former half), the base signalperiodically oscillates at an essentially identical cycle with acomparatively long wavelength. In the presence of the reverse flow (asseen from the latter half), the waveform of the signal from the air-flowmeter is constructed by a medium ridge (corresponding to a forward airflow) and a small ridge (corresponding to a reverse air flow) combinedwith each other, since the air-flow meter detects and outputs thereverse-flow component as a positive signal value. By way of the routineshown in FIG. 67, the minimal state (Flg₋₋ min=1) and the maximal state(Flg₋₋ max=1) are detected. By way of the routine shown in FIG. 68, thesignal-value increasing time interval C₋₋ Inc and the signal-valuedecreasing time interval C₋₋ Dec are measured. As seen from thewaveforms of the upper three signals, namely the base signal, thesignal-value decreasing time interval indicative signal (C₋₋ Dec) andthe signal-value increasing time interval indicative signal (C₋₋ Inc),at the timing (as marked by the black dot) of decision of the maximalvalue, i.e., when the condition (Flg₋₋ max=1) is satisfied, themeasurement for the signal-value decreasing time interval C₋₋ Decbegins, and this measurement continues until the minimal value isreached from the maximal value, i.e., the flag Flg₋₋ min is set at "1".As soon as the minimal value is reached, the signal-value decreasingtime interval C₋₋ Dec is updated by a newly measured time interval.Similarly, at the timing (as marked by the small circle) of decision ofthe minimal value, i.e., when the condition (Flg₋₋ min=1) is satisfied,the measurement for the signal-value increasing time interval C₋₋ Incbegins, and this measurement continues until the maximal value isreached from the minimal value, i.e., the flag Flg₋₋ max is set at "1".As soon as the maximal value is reached, the signal-value increasingtime interval C₋₋ Inc is updated by a newly measured time interval. InFIG. 69, the signal DC denotes the deviation between the signal-valuedecreasing time interval indicative signal C₋₋ Dec and the signal-valueincreasing time interval indicative signal C₋₋ Inc, and the shaded zonesshow respective signal-processed zones or inverted signal zones whichcan be determined depending on the comparison of the decreasing timeinterval C₋₋ Dec and the increasing time interval C₋₋ Inc, preciselydepending on the deviation DC (=C₋₋ Dec-C₋₋ Inc). The signal-processingis actually based on the flow chart shown in FIG. 70. The sub-routineshown in FIG. 70 is cooperative with the two sub-routines discussed inFIGS. 67 and 68 so as to decide the presence of the reverse-flow and thetime interval of the reverse flow and timely precisely set asignal-inversion flag Flg₋₋ neg at "1", as explained later.

Referring to FIG. 70, in step S431, calculated by the expression (DC=C₋Dec₋ C₋₋ Inc) is the deviation DC between the current value of thedecreasing time interval C₋₋ Dec and the current value of the increasingtime interval C₋₋ Inc. In step S432, a test is made to determine whetherthe current deviation DC is a positive number, that is DC>0. When theanswer to step S432 is affirmative, step S433 proceeds in which thesignal-inversion flag Flg₋₋ neg is set at "1". When the answer to stepS432 is negative, step S434 proceeds in which a test is made todetermine whether the maximal-state indicative flag Flg₋₋ max and thesignal-inversion flag Flg₋₋ neg are both set at "1". When the answer tostep S434 is affirmative, step S435 enters so that the signal-inversionflag Flg₋₋ neg is reset to zero. When the answer to step S434 isnegative, step S436 proceeds in which a test is made to determinewhether the previous value DC(n-1) of the deviation is a negative numberand additionally the minimal-state indicative flag Flg₋₋ min is "1".When the answer to step S436 is affirmative (YES), the procedure flowsto step S433 so as to set the flag Flg₋₋ neg at "1". In contrast to theabove, when the answer to step S436 is negative (NO), step S437 proceedsin which the previous value of the flag Flg₋₋ neg is regarded as thecurrent value of the flag Flg-neg, that is, the previous value of theflag Flg-neg remains unchanged through the current routine. As may beappreciated from the flow chart of FIG. 70, the control unit decides thepresence of the reverse flow and then the signal-inversion flag Flg₋₋neg is set at "1", in the case that the current value of the deviationDC is a positive number or for instance as appreciated from the shadedzones of FIG. 69 in the case that the deviation DC is kept at zeroduring the time period from the time when the deviation DC is changedfrom the negative number to zero and additionally the minimal valuebecomes reached to the time when a next maximal value is reached.Therefore, the previously-noted averaging process is performed on thebasis of the converted or inverted signal of FIG. 69, to produce thefinal fresh-air flow rate indicative signal (Qas0), thereby ensuring ahigh-precision EGR control and a high-precision fuel-injection amountcontrol (or a high-precision air-fuel-ratio control).

FIGS. 71A through 71C show test results similar to the test shown inFIG. 65, in case of the signal-processing made to the signal output fromthe hot-wire type air-flow meter for the reverse-flow correction relatedto the flow charts shown in FIGS. 67, 68 and 70. FIGS. 71A, 71B and 71Crespectively indicate the waveform of the signal output from theair-flow meter, the waveform of the signal advance-processed, and thewaveform of the signal properly converted or inverted through thereverse-flow correction. On the other hand, FIG. 72 shows simulationresults in case that the system of the fifth embodiment is applied to adiesel engine with an EGR control system as shown in FIGS. 1 and 29. Thesimulation test was performed by the inventor of the present inventionunder a condition in which the amount of exhaust-gas recirculation (EGR)gradually increases in accordance with an increase in elapsed timeduring idling so that the actual induced fresh-air flow rate measured bythe hot-wire air-flow meter gradually reduces. Additionally, in thetest, the engine speed is kept at 850 rpm and the sampling period oftime is set at 1 msec, and induced fresh-air pulsation flow is appliedin the form of a sinusoidal wave. As seen in FIG. 72, the actualintake-air flow rate, a flow-rate indicative signal obtained through theprior art system, and a properly signal-processed flow-rate indicativesignal obtained through the flow shown in FIGS. 67, 68 and 70 aresubstantially same within a comparatively higher flow-rate range (untilthe elapsed time 14 sec). Within a comparatively lower flow-rate range(within the time interval from the elapsed time 22 sec to the elapsedtime 24 sec), the signal-processed flow-rate indicative signal obtainedthrough the present invention matches the actual flow rate, although theflow-rate indicative signal obtained through the prior art system isoutput as a signal level considerably higher than the actual flow rate.As will be appreciated from the above, the system of the fifthembodiment utilizes a typical inexpensive hot-wire air-flow meter, thusreducing total production costs of the integrated engine control systemof the invention. Additionally, as set out above, the system of thefifth embodiment can insure a high-precision reverse-flow detection andensure a high-precision induced air flow detection through the advanceprocessing and the reverse-flow correction (the signal processing).

While the foregoing is a description of the preferred embodimentscarried out the invention, it will be understood that the invention isnot limited to the particular embodiments shown and described herein,but that various changes and modifications may be made without departingfrom the scope or spirit of this invention as defined by the followingclaims.

What is claimed is:
 1. An integrated internal combustion engine controlsystem in combination with an automotive emission control system, saidengine control system comprising:an exhaust-gas recirculation valveemployed in an exhaust-gas recirculation system for sending some ofexhaust gas back through an internal combustion engine; air flow ratedetection means for detecting a flow rate of intake air drawn into theengine; engine-operating-condition detection means for detecting anoperating condition of the engine; target exhaust-gas recirculation ratesetting means for setting a target exhaust-gas recirculation ratedepending on said operating condition of the engine; target exhaust-gasrecirculation amount setting means for setting a target exhaust-gasrecirculation amount as a function of said flow rate of intake air andsaid target exhaust-gas recirculation rate; and valve-operating controlmeans responsive to said target exhaust-gas recirculation amount forcontrolling an opening of said exhaust-gas recirculation valve, whereinsaid valve-operating control means comprises a command exhaust-gasrecirculation amount setting means for setting a command exhaust-gasrecirculation amount (Tqec) by performing a predetermined advanceprocessing with respect to said target exhaust-gas recirculation amount(Tqec0), and a controlled variable setting means for setting acontrolled variable (Liftt) of the opening of said exhaust-gasrecirculation valve depending on said command exhaust-gas recirculationamount, and said command exhaust-gas recirculation amount setting meanscomprises a response-characteristic time constant setting means forsetting a response characteristic between a target exhaust-gasrecirculation amount (MQce) which is intended to be drawn into thecylinder and an actual exhaust-gas recirculation amount (Qce) which isactually drawn into the engine cylinder, and a dynamic-characteristictime constant estimation means for estimating dynamic characteristics ofa recirculated exhaust gas, flowing through said exhaust-gasrecirculation valve into the engine cylinder, and wherein said commandexhaust-gas recirculation amount which is intended to be passed throughsaid exhaust-gas recirculation valve is set by making said predeterminedadvance processing to said target exhaust-gas recirculation amount(MQce) in accordance with said dynamic characteristics estimated by saiddynamic-characteristic time constant estimation means so that saidresponse characteristic set by said response-characteristic timeconstant setting means is reached.
 2. The integrated internal combustionengine control system as set forth in claim 1, wherein saiddynamic-characteristic time constant estimation means estimates adynamic-characteristic time constant (τa) from an engine speed (Ne), avolumetric efficiency (ηv), and a volumetric capacity in an inductionsystem from said exhaust-gas recirculation valve to an inlet of anengine cylinder and a volumetric capacity of the engine cylinder, andwherein said response-characteristic time constant setting means sets aresponse-characteristic time constant (τs) at a positive number lessthan said dynamic-characteristic time constant (τa).
 3. The integratedinternal combustion engine control system as set forth in claim 2, whichfurther comprises an intake-pressure detection means for detecting anintake pressure (Pm) and an exhaust-pressure detection means fordetecting an exhaust pressure (Pexh), and said controlled variablesetting means calculates a required fluid-flow passage area (Tav)defined by said exhaust-gas recirculation valve as a function of saidcommand exhaust-gas recirculation amount (Tqec), said intake pressure(Pm) and said exhaust pressure (Pexh), and sets a target opening (Mlift)of said exhaust-gas recirculation valve so that said target opening(Mlift) is equivalent to said required flow passage area (Tav), andcontrolled variable setting means including an advance-processing meansthrough which said controlled variable (Liftt) is set by making apredetermined advance processing to said target opening (Mlift).
 4. Anintegrated internal combustion engine control system with an automotiveemission control system, said engine control system comprising:anexhaust-gas recirculation valve employed in an exhaust-gas recirculationsystem for sending back some of exhaust gas through an internalcombustion engine; an air flow meter for detecting a flow rate of intakeair drawn into the engine; an engine-operating-condition detection meansfor detecting an operating condition of the engine; a target exhaust-gasrecirculation amount setting means for setting a target exhaust-gasrecirculation amount depending on both said operating condition of theengine and said flow rate of intake air; and an exhaust-gasrecirculation control means for controlling an opening of saidexhaust-gas recirculation valve in response to said target exhaust-gasrecirculation amount, wherein said exhaust-gas recirculation controlmeans comprises a pressure estimation means for estimating an intakepressure and an exhaust pressure from the intake-air flow rate measuredby said air flow meter, a differential-pressure estimation means forestimating a differential pressure as the difference between the exhaustand intake pressures, a response-delay estimation means for estimating,depending on the differential pressure, a response delay between atarget exhaust-gas recirculation amount which is intended to be drawninto the cylinder and an actual exhaust-gas recirculation amount whichis actually drawn into the cylinder, and an advance-processing means foradvance-compensating for the target exhaust-gas recirculation amount bya predetermined advance processing.
 5. The integrated internalcombustion engine control system as set forth in claim 4, wherein saidadvance-processing means comprises a command exhaust-gas recirculationamount setting means for setting a command exhaust-gas recirculationamount by performing the predetermined advance processing with respectto said target exhaust-gas recirculation amount, and a controlledvariable setting means for setting a controlled variable of the openingof said exhaust-gas recirculation valve depending on the commandexhaust-gas recirculation amount, and said response-delay estimationmeans comprises a response-characteristic time constant setting meansfor setting a response characteristic between the target exhaust-gasrecirculation amount and the actual exhaust-gas recirculation amount,and a dynamic-characteristic time constant estimation means forestimating dynamic characteristics of a recirculated exhaust gas,flowing through said exhaust-gas recirculation valve into the enginecylinder, and wherein said command exhaust-gas recirculation amountwhich is intended to be passed through said exhaust-gas recirculationvalve is set by making said predetermined advance processing to thetarget exhaust-gas recirculation amount in accordance with said dynamiccharacteristics estimated by said dynamic-characteristic time constantestimation means so that the response characteristic set by saidresponse-characteristic time constant setting means is reached.
 6. Theintegrated internal combustion engine control system as set forth inclaim 5, wherein said dynamic-characteristic time constant estimationmeans estimates a dynamic characteristic time constant from an enginespeed, a volumetric efficiency, and a volumetric capacity in aninduction system from said exhaust-gas recirculation valve to an inletof an engine cylinder and a volumetric capacity of the engine cylinder,and wherein said response-characteristic time constant setting meanssets a response-characteristic time constant at a positive number lessthan said dynamic-characteristic time constant.
 7. The integratedinternal combustion engine control system as set forth in claim 6,wherein said controlled variable setting means calculates a requiredfluid-flow passage area defined by said exhaust-gas recirculation valveas a function of said command exhaust-gas recirculation amount, saidintake pressure, and said exhaust pressure (Pexh), to set a target liftof said exhaust-gas recirculation valve so that said target lift isequivalent to said required fluid-flow passage area, and saidexhaust-gas recirculation control means performs the predeterminedadvance processing with respect to said target lift to obtain saidcontrolled variable.
 8. The integrated internal combustion enginecontrol system as set forth in claim 5, which further comprises a liftdetection means for detecting a lift of a valve stem of said exhaust-gasrecirculation valve, and wherein the opening of the exhaust-gasrecirculation valve is controlled in response to the advance-processedtarget lift so that the lift detected by said lift detection means isadjusted toward the target lift, and which further comprises a throttlevalve control means for controlling a throttle valve depending on saidintake pressure, said exhaust pressure, and the lift detected by saidlift detection means.
 9. The integrated internal combustion enginecontrol system as set forth in claim 8, wherein saiddifferential-pressure estimation means calculates a maximum recirculatedexhaust-gas flow rate as a function of said differential pressure and amaximum opening area of said exhaust-gas recirculation valve, andwherein said throttle valve control means decreasingly adjusts anopening of said throttle valve when a desired recirculated exhaust-gasflow rate exceeds said maximum recirculated exhaust-gas flow rate. 10.The integrated internal combustion engine control system as set forth inclaim 9, wherein said throttle valve control means increasingly adjuststhe opening of said throttle valve when said desired recirculatedexhaust-gas flow rate is below said maximum recirculated exhaust-gasflow rate and additionally said differential pressure is above apredetermined slice level.
 11. The integrated internal combustion enginecontrol system as set forth in claim 9, wherein said throttle valvecontrol means increasingly adjusts the opening of said throttle valvewhen said desired recirculated exhaust-gas flow rate is below saidmaximum recirculated exhaust-gas flow rate and additionally said liftdetected by said lift detection means is less than a predetermined slicelevel.
 12. The integrated internal combustion engine control system asset forth in claim 9, wherein said throttle valve control meansincreasingly adjusts the opening of said throttle valve when saiddesired recirculated exhaust-gas flow rate is below said maximumrecirculated exhaust-gas flow rate, said differential pressure is abovea predetermined slice level, and additionally said lift detected by saidlift detection means is less than a predetermined slice level.